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darker_blackRegulus was born at home. It wasn't that uncommon for witches to give birth at home, but part of the reason was that Walburga was having one of her rather bad days and had refused to leave the house.
Unlike I've seen in some fics, this Regulus's parents were never physically abusive and they never would have used Crucio on their children (especially since that can cause permanent harm). Verbally and emotionally abusive is another matter. Walburga Black had an explosive temper while Orion Black was strict, cold, and distant. Regulus thought this was perfectly normal, and could never grasp why Sirius came to think their parents didn't care about them. (Sirius had been exposed to more normal parents like James Potter's) Either Regulus had a talent for reading between the lines of his parents' actions and understanding their motivations, or he was deluding himself into seeing what he wanted to see in them. Given a similar talent expressed later, it's very possible it's the former.
To teach him to write properly with a quill, Regulus's mother had him copy out sections of their family tree. He can still do this from memory for at least some of the various branches, especially the more recent ones. (read as any actually confirmed in canon, so I don't have to make stuff up there)
Regulus has often had trouble sleeping, even when he was a child. When he was very small he was afraid of thunderstorms. There were times, mostly when he was about five years old, when he would sneak into Sirius's room when the storm woke him and climb in bed with his brother. Back then, he felt safe if Sirius was there.
When they were older and sorted into seperate houses, Regulus would sneak down to the kitchen on nights when he couldn't sleep. Kreacher would always fix him hot chocolate. In later years the hot chocolate was often laced with a calming draught or some other potion to help Regulus get back to sleep... after his school years, the chocolate itself just wasn't enough to keep away the nightmares.
The sorting hat considered putting Regulus in Ravenclaw, but his Slytherin traits are stronger and he didn't want to disappoint his parents. Sometimes he wishes he'd chosen differently, as he thinks being sorted into houses prone to a very strong rivalry played a good part in destroying his relationship with his older brother.
Regulus can't cast a Patronus. He's not very good with Crucio either, but he's actually somewhat relieved about that.
He'd try to hide this around most of his former crowd, but his cousin Bellatrix really scares him.
Regulus remembers an incident, sometime when he was home for the summer after his second year and Sirius's third year. James Potter had come to visit. For a bit of entertainment, James and Sirius took to tormenting Kreacher. When Regulus had found out he'd actually stepped in to try to protect the house elf, and James Potter was quite willing to beat up the little snake for it. Before Sirius really had time to consider which side of the resulting disaster he should be on, Walburga caught them. James Potter was never again welcome at Grimmauld Place, though Sirius was still allowed - albeit grudgingly - to visit him at his own home. Regulus suspects that later this was another thing Sirius blamed him for, that James couldn't come over because of Regulus and 'that stupid elf'.
He has a dislike for James Potter that mostly stems from feeling like James stole Sirius. That a few teachers had commented over the years that James and Sirius were 'as close as brothers' probably did NOT help this.
House elves can't be ordered to kill themselves, but even then Regulus understood that there are worse things than death. He understood, even when very young, that Sirius would inherit, and that Sirius did not like Kreacher. He'd had a lot of talks with Kreacher about various things and, after the incident with James Potter, insisted on putting a 'precautionary' plan into effect. It was actually a very simple loophole to exploit. Regulus took a thin scarf from his wardrobe. With Kreacher there, and explaining the plan very carefully to the old house elf, he placed the scarf in a small wooden box and they concealed it in the kitchen. The scarf wasn't 'given' to Kreacher... unless Kreacher opened the box. Anyone else opening the box didn't count, and Kreacher knew exactly what was in the box. The two went over the plan carefully to check for complications before carrying it out. That way, when Sirius inherited the house, if Regulus wasn't around or for some other reason couldn't stop him from being horrible to Kreacher, the old house elf had a way to escape - despite that, at the time, Kreacher insisted he would never use it. (Much later, one of the first rooms the Order cleaned was the kitchen. Molly Weasley found the box and removed the scarf during this. She never knew what purpose it was meant to serve, but this left Kreacher without the plan that he might otherwise have used instead of participating in the plot that lead to Sirius's death.)
He never really hated muggle-borns as much as some of the others seemed to, though he'd go through the motions as expected, using the same slurs as the rest of the crowd he was with. Better not to stand out too much. In truth, he almost felt sorry for them, shoved into a world they hadn't been brought up to understand, but that just wasn't the sort of thing one said around his family. He even had some slightly grudging respect for Lily Evans, noting that she seemed very talented despite her unfortunate ancestry.
If he'd been caught even a tenth of the times he'd done it, Regulus would have had a reputation for evesdropping in school. He was only caught a handful of times... He probably has more dirt on some people than Bathilda Bagshot had on Dumbledore, but he tends to keep all secrets he's learned to himself. If it wouldn't benefit him in some way to do so, he sees no point in telling, and he's not (usually) petty enough to do so just for amusement. He just likes knowing things and, given the old conversation with the sorting hat, he suspects this is how Ravenclaw traits express themselves in a Slytherin.
Regulus is good at keeping things to himself.
His parents actions regarding Sirius's behavior hurt him, but he thought he understood. With the way Sirius was acting, something had to be done. Publicly disowning him, with Walburga even going so far as to blast Sirius off the tapestry, and Orion making sure Regulus was named heir in Sirius's place... To Regulus, it was a case of trying to show Sirius what he was doing, in the hopes that seeing how much it hurt to have your family reject you might get Sirius to stop doing it himself. That this backfired massively made Regulus question whether Sirius cared about any of them at all.
He knows, after all, that Sirius's room remained untouched, waiting in case he ever came back. And he knows that Sirius wasn't legally disowned... that if anything ever happened to Regulus, Sirius would still inherit. If he'd been disowned, it would have gone to cousin Bellatrix. And knowing that was something of a comfort when he was certain he was going to die, as was the thought that maybe, just maybe, Sirius would figure it out someday, and know that they always wanted him to come home.
Regulus never really dated much in school, outside of what was expected and certain social functions. He'd accepted early on that he'd probably end up in arranged marriage, and was afraid of taking a chance of falling in love and thus making things harder on himself. That girls scare him a bit (due to Cousin Bellatrix) possibly also contributed to this.
Regulus probably kept getting asked from his first year at school what he thought of various 'acceptable' girls so his parents could plan for such a thing for him, though that particular pressure wasn't too heavy until after Sirius left and he was named heir in his brother's place. It's actually no small miracle he wasn't already married shortly after school, but his parents had agreed to wait to arrange a betrothal until he was of age... and then he was Marked the year before he came of age. As the stress wore on him, he begged for that to be set aside until after the war, and, in a rare moment of softness for that family, his parents had agreed... though they had reminded him rather sharply of his duty to his family and hoped he'd keep looking for an acceptable mate.
As for Sirius, Regulus didn't even try to keep up with all the girls his brother dated. Sometimes he thought Sirius and James were in some kind of competition to see who could snog the most girls in a year. He found it a little disgusting, to be honest. He does remember Sirius having an on-and-off relationship with Marlene McKinnon in the last couple years Sirius was in school, but has no idea where that might have gone. And, of course, he couldn't know Marlene was killed in 1981, as he was 'dead' by then.
He wasn't even of legal age when he was marked, instead being sixteen (as his first cousin once removed Draco later would be). He was reminded then that Sirius would have been old enough... as if he were there to take the place his brother had rejected. Sometimes he wished he could be himself, instead of a 'backup because Sirius ran away'... but he wasn't really sure who he was other than what others wanted him to be. (Most teenagers aren't, leading to much rebellious angst. Regulus just went the other way, conforming instead of rebelling)
He didn't really keep up with what certain people were doing after school, and would be shocked to hear of Lily Evans marrying James Potter. He does recall them dating, but he also recalls Lily punching James in the Great Hall during his sixth year (their 7th) and rumors of love potion use. He really had too many other things on his mind by then, though, to check into anything and see what was really going on. Potter's love life was not a priority.
He's disgusted by the things he's seen and done. Moreso that it took him so long to dare to question. He knew things weren't right, but kept trying to convince himself there had to be a reason... no matter how much it looked like the petty bullying in school, taken to a grander and more lethal scale.
He thought he knew what at least some of the goals were. He learned he was mistaken about some, and unaware of others. And very much in over his head.
He did not rush to the cave as soon as he learned about it. That would have been a foolish and Gryffindor thing to do. He was angry that Kreacher had been left to die in the cave and had only been saved because of Regulus's order for him to return, but didn't want to tip his hand until he knew exactly what was going on. The incident was what prompted him to doubt the Dark Lord and start a quiet investigation of his own, though. Besides, poor Kreacher was sick for nearly four days and Regulus had to look after him and make sure his parents didn't find out, as well as making sure the Dark Lord never learned that Kreacher had survived - luckily Voldemort never cared enough to ask about that.
After the incident with Kreacher, Regulus did a lot of careful checking things out. He noticed some books missing from the Malfoys when he went to visit his cousin Narcissa, and found a few gone from the Black library as well. Kreacher was able to tell him what some of the books were, and from there he was able to track enough info down to put together with what Kreacher had told him. He came to the conclusion that the Dark Lord had ingratiated himself to the various older pureblood houses to gain access to those books, in order to learn to make a horcrux. He never realized that the Dark Lord already knew how to make horcruxes and was instead trying to conceal the knowledge of how to destroy them. (Which is why certain books had been 'missed' that let Regulus learn what a horcrux was, but he couldn't learn for certain how to destroy one and be able to tell the proper method to Kreacher.) He also never knew Voldemort had already made more than one of the things.
Around that time, while going through so many books, Regulus also learned that the last known descendants of Salazar Slytherin were a family named Gaunt. Given that Voldemort had claimed to be Slytherin's heir at some point, he managed to piece together that muddle and shock himself by learning who Voldemort had been... and that he was the bastard child of a muggle and a near-squib. (Or at least Regulus considers him illegitimate. Even though Merope Gaunt and Tom Riddle had married, a marriage while under the effects of a love potion probably wouldn't be considered binding or would be considered very good grounds for dissolution)
The sheer hypocrisy of the whole thing made Regulus's little world crumble. He also suspects Tom was likely secretly very amused at being able to practically make slaves out of the children of those who would have looked down him in school for his ancestry. (If they'd known it. They did know he was returned to a Muggle orphanage over the summers...)
Regulus does not easily lose his temper, raise his voice, or use profanity. He's far more the 'don't get mad, get even' sort. However there are some things that will cross the line with him. An identified danger to his family is the top of that very short list. Even when he's angry, though, he takes after his father in usually remaining outwardly calm and very calculating. (Sirius, instead, seemed to have inherited their mother's temper) In this (rare) mood, he can even be cruel, though he would probably regret that later when he's calmed down. It's unlikely any of his school 'friends' would have ever seen him like this, as Regulus would more than likely excuse himself and leave a room rather than get upset and make some kind of inappropriate scene. It's even possible nothing really seemed enough of a threat to trigger this reaction until Voldemort. Even after he'd calmed down, the resolution remained in place to find a way to make Voldemort regret thinking he could just use the House of Black...
Peter Pettigrew is another topic that can trigger a rare angry Regulus. He hated the Marauders in school for being Sirius's 'replacement family', but at the same time that was always tinged with some hurt as it was Sirius's choice and he could tell his brother genuinely cared about them. He'd overheard some discussion of others among his old acquaintances who were grooming a 'weak link' from the former 'Marauders' to turn spy. It wasn't hard to guess who they meant from some things said, and he has no doubt they'd get Peter to turn, if he hadn't already. It infuriates him to the point where he can't even see any irony in the 'family' that his brother abandoned him for turning around and betraying him. He hates Peter for being a threat to Sirius, who, in his heart, he still considers his brother. But, strangely enough, he also hates Peter for being a threat to the Marauders, who seemed like closer 'family' than his own was, and who Regulus knows probably - with Peter as exception - would have died for each other. That's the kind of family Regulus really only had in his dreams, the kind he wishes his family could be, and to think of Peter throwing it away... well... if Regulus ever catches Pettigrew, he might actually be able to cast a proper Crucio for once in his life.
As he made his plans it started to get more clear to him that he wasn't going to survive. If he did, he would be thought a traitor, and probably tortured to death, possibly along with his parents if the Dark Lord was in a particularly cruel mood. If he went into hiding, then his family would likely be tortured and possibly killed to draw him out. Dying was the only way he could think of to keep them safe. Somewhere during the bargaining stage of this realization, with his plans already mostly formed as far as dealing with the locket, he took the chance to write to Sirius, asking his brother to meet him in secret. He had the last, desperate hope that maybe his brother would think of some way out that he'd missed. As far as Regulus knows, Sirius burned the letter.
He spent a good part of the last day of his life dressed as a muggle (having transfigured his robes for a disguise), in the park he'd asked Sirius to meet him in. It was chilly, but not snowing, and for most of the day he was the only one there... except what seemed to be a large black stray dog that he ignored after an uncomfortable moment of noting that it looked rather like a Grim. He may have commented out loud, and a little bitterly, that that was appropriate. Other than that he didn't speak for most of the day. When the sun went down and the stars started to come out, he gave up on his brother.
He said his goodbyes instead to the star his brother had been named for, once it appeared in the sky, before apparating away. (Hinting that he expected to die soon, but not mentioning the horcrux or his suspicions about Peter since he thought he was essentially talking to himself. Sirius had thought for sure it was a trap, and, tragically, decided to reveal himself just a split second too late. This is what left him with the idea that Regulus had tried to back out and was killed for it - It's not a story I think Voldemort would have tried to spread on his own, given the rest of Regulus's family (sans Sirius) seemed loyal, it would have been better strategy to blame Regulus's disappearance on the Order. And, yes, I know the book has Sirius say that's based on stuff he found out after Regulus died, but... he probably wouldn't have really pieced together all of it until Regulus had been missing long enough to be considered dead.)
Even if he could later learn about Padfoot, he might not make the connection to the dog seen in the park - given the circumstances, he was almost certain that was a Grim.
He went to the cave that same night, concerned his resolve would falter if he waited any longer. He'd gone over a few things with Kreacher before leaving, but didn't tell the house elf most of the details until they'd arrived in the cave. Kreacher protested - for the first and only time in Regulus's memory, at least regarding anything he'd asked of the old elf - but at Regulus's insistence Kreacher did as he was told... including taking the locket and leaving Regulus behind to the inferi.
He (almost) died in early winter (November), not long after his cousin Narcissa let the rest of the family know she was pregnant. He was 18 years old.
Peter Pettigrew turned spy a short time later.
Regulus's father, Orion Black, died that same year under suspicious circumstances, as did his uncle Cygnus Black. Orion never believed the official Death Eater story that the Order - specifically Sirius - had killed Regulus, and after placing numerous additional defensive enchantments on the family's already well-secured home, he left to confront his oldest son about the matter. Not being a fool, he hadn't gone without backup, asking his wife's brother, Cygnus, to accompany him. Unfortunately, Cygnus mentioned the impending visit to a member of his own wife's family (Rosier). Neither one returned from going to see Sirius, and death dates appeared on the family tapestry.
Unlike I've seen in some fics, this Regulus's parents were never physically abusive and they never would have used Crucio on their children (especially since that can cause permanent harm). Verbally and emotionally abusive is another matter. Walburga Black had an explosive temper while Orion Black was strict, cold, and distant. Regulus thought this was perfectly normal, and could never grasp why Sirius came to think their parents didn't care about them. (Sirius had been exposed to more normal parents like James Potter's) Either Regulus had a talent for reading between the lines of his parents' actions and understanding their motivations, or he was deluding himself into seeing what he wanted to see in them. Given a similar talent expressed later, it's very possible it's the former.
To teach him to write properly with a quill, Regulus's mother had him copy out sections of their family tree. He can still do this from memory for at least some of the various branches, especially the more recent ones. (read as any actually confirmed in canon, so I don't have to make stuff up there)
Regulus has often had trouble sleeping, even when he was a child. When he was very small he was afraid of thunderstorms. There were times, mostly when he was about five years old, when he would sneak into Sirius's room when the storm woke him and climb in bed with his brother. Back then, he felt safe if Sirius was there.
When they were older and sorted into seperate houses, Regulus would sneak down to the kitchen on nights when he couldn't sleep. Kreacher would always fix him hot chocolate. In later years the hot chocolate was often laced with a calming draught or some other potion to help Regulus get back to sleep... after his school years, the chocolate itself just wasn't enough to keep away the nightmares.
The sorting hat considered putting Regulus in Ravenclaw, but his Slytherin traits are stronger and he didn't want to disappoint his parents. Sometimes he wishes he'd chosen differently, as he thinks being sorted into houses prone to a very strong rivalry played a good part in destroying his relationship with his older brother.
Regulus can't cast a Patronus. He's not very good with Crucio either, but he's actually somewhat relieved about that.
He'd try to hide this around most of his former crowd, but his cousin Bellatrix really scares him.
Regulus remembers an incident, sometime when he was home for the summer after his second year and Sirius's third year. James Potter had come to visit. For a bit of entertainment, James and Sirius took to tormenting Kreacher. When Regulus had found out he'd actually stepped in to try to protect the house elf, and James Potter was quite willing to beat up the little snake for it. Before Sirius really had time to consider which side of the resulting disaster he should be on, Walburga caught them. James Potter was never again welcome at Grimmauld Place, though Sirius was still allowed - albeit grudgingly - to visit him at his own home. Regulus suspects that later this was another thing Sirius blamed him for, that James couldn't come over because of Regulus and 'that stupid elf'.
He has a dislike for James Potter that mostly stems from feeling like James stole Sirius. That a few teachers had commented over the years that James and Sirius were 'as close as brothers' probably did NOT help this.
House elves can't be ordered to kill themselves, but even then Regulus understood that there are worse things than death. He understood, even when very young, that Sirius would inherit, and that Sirius did not like Kreacher. He'd had a lot of talks with Kreacher about various things and, after the incident with James Potter, insisted on putting a 'precautionary' plan into effect. It was actually a very simple loophole to exploit. Regulus took a thin scarf from his wardrobe. With Kreacher there, and explaining the plan very carefully to the old house elf, he placed the scarf in a small wooden box and they concealed it in the kitchen. The scarf wasn't 'given' to Kreacher... unless Kreacher opened the box. Anyone else opening the box didn't count, and Kreacher knew exactly what was in the box. The two went over the plan carefully to check for complications before carrying it out. That way, when Sirius inherited the house, if Regulus wasn't around or for some other reason couldn't stop him from being horrible to Kreacher, the old house elf had a way to escape - despite that, at the time, Kreacher insisted he would never use it. (Much later, one of the first rooms the Order cleaned was the kitchen. Molly Weasley found the box and removed the scarf during this. She never knew what purpose it was meant to serve, but this left Kreacher without the plan that he might otherwise have used instead of participating in the plot that lead to Sirius's death.)
He never really hated muggle-borns as much as some of the others seemed to, though he'd go through the motions as expected, using the same slurs as the rest of the crowd he was with. Better not to stand out too much. In truth, he almost felt sorry for them, shoved into a world they hadn't been brought up to understand, but that just wasn't the sort of thing one said around his family. He even had some slightly grudging respect for Lily Evans, noting that she seemed very talented despite her unfortunate ancestry.
If he'd been caught even a tenth of the times he'd done it, Regulus would have had a reputation for evesdropping in school. He was only caught a handful of times... He probably has more dirt on some people than Bathilda Bagshot had on Dumbledore, but he tends to keep all secrets he's learned to himself. If it wouldn't benefit him in some way to do so, he sees no point in telling, and he's not (usually) petty enough to do so just for amusement. He just likes knowing things and, given the old conversation with the sorting hat, he suspects this is how Ravenclaw traits express themselves in a Slytherin.
Regulus is good at keeping things to himself.
His parents actions regarding Sirius's behavior hurt him, but he thought he understood. With the way Sirius was acting, something had to be done. Publicly disowning him, with Walburga even going so far as to blast Sirius off the tapestry, and Orion making sure Regulus was named heir in Sirius's place... To Regulus, it was a case of trying to show Sirius what he was doing, in the hopes that seeing how much it hurt to have your family reject you might get Sirius to stop doing it himself. That this backfired massively made Regulus question whether Sirius cared about any of them at all.
He knows, after all, that Sirius's room remained untouched, waiting in case he ever came back. And he knows that Sirius wasn't legally disowned... that if anything ever happened to Regulus, Sirius would still inherit. If he'd been disowned, it would have gone to cousin Bellatrix. And knowing that was something of a comfort when he was certain he was going to die, as was the thought that maybe, just maybe, Sirius would figure it out someday, and know that they always wanted him to come home.
Regulus never really dated much in school, outside of what was expected and certain social functions. He'd accepted early on that he'd probably end up in arranged marriage, and was afraid of taking a chance of falling in love and thus making things harder on himself. That girls scare him a bit (due to Cousin Bellatrix) possibly also contributed to this.
Regulus probably kept getting asked from his first year at school what he thought of various 'acceptable' girls so his parents could plan for such a thing for him, though that particular pressure wasn't too heavy until after Sirius left and he was named heir in his brother's place. It's actually no small miracle he wasn't already married shortly after school, but his parents had agreed to wait to arrange a betrothal until he was of age... and then he was Marked the year before he came of age. As the stress wore on him, he begged for that to be set aside until after the war, and, in a rare moment of softness for that family, his parents had agreed... though they had reminded him rather sharply of his duty to his family and hoped he'd keep looking for an acceptable mate.
As for Sirius, Regulus didn't even try to keep up with all the girls his brother dated. Sometimes he thought Sirius and James were in some kind of competition to see who could snog the most girls in a year. He found it a little disgusting, to be honest. He does remember Sirius having an on-and-off relationship with Marlene McKinnon in the last couple years Sirius was in school, but has no idea where that might have gone. And, of course, he couldn't know Marlene was killed in 1981, as he was 'dead' by then.
He wasn't even of legal age when he was marked, instead being sixteen (as his first cousin once removed Draco later would be). He was reminded then that Sirius would have been old enough... as if he were there to take the place his brother had rejected. Sometimes he wished he could be himself, instead of a 'backup because Sirius ran away'... but he wasn't really sure who he was other than what others wanted him to be. (Most teenagers aren't, leading to much rebellious angst. Regulus just went the other way, conforming instead of rebelling)
He didn't really keep up with what certain people were doing after school, and would be shocked to hear of Lily Evans marrying James Potter. He does recall them dating, but he also recalls Lily punching James in the Great Hall during his sixth year (their 7th) and rumors of love potion use. He really had too many other things on his mind by then, though, to check into anything and see what was really going on. Potter's love life was not a priority.
He's disgusted by the things he's seen and done. Moreso that it took him so long to dare to question. He knew things weren't right, but kept trying to convince himself there had to be a reason... no matter how much it looked like the petty bullying in school, taken to a grander and more lethal scale.
He thought he knew what at least some of the goals were. He learned he was mistaken about some, and unaware of others. And very much in over his head.
He did not rush to the cave as soon as he learned about it. That would have been a foolish and Gryffindor thing to do. He was angry that Kreacher had been left to die in the cave and had only been saved because of Regulus's order for him to return, but didn't want to tip his hand until he knew exactly what was going on. The incident was what prompted him to doubt the Dark Lord and start a quiet investigation of his own, though. Besides, poor Kreacher was sick for nearly four days and Regulus had to look after him and make sure his parents didn't find out, as well as making sure the Dark Lord never learned that Kreacher had survived - luckily Voldemort never cared enough to ask about that.
After the incident with Kreacher, Regulus did a lot of careful checking things out. He noticed some books missing from the Malfoys when he went to visit his cousin Narcissa, and found a few gone from the Black library as well. Kreacher was able to tell him what some of the books were, and from there he was able to track enough info down to put together with what Kreacher had told him. He came to the conclusion that the Dark Lord had ingratiated himself to the various older pureblood houses to gain access to those books, in order to learn to make a horcrux. He never realized that the Dark Lord already knew how to make horcruxes and was instead trying to conceal the knowledge of how to destroy them. (Which is why certain books had been 'missed' that let Regulus learn what a horcrux was, but he couldn't learn for certain how to destroy one and be able to tell the proper method to Kreacher.) He also never knew Voldemort had already made more than one of the things.
Around that time, while going through so many books, Regulus also learned that the last known descendants of Salazar Slytherin were a family named Gaunt. Given that Voldemort had claimed to be Slytherin's heir at some point, he managed to piece together that muddle and shock himself by learning who Voldemort had been... and that he was the bastard child of a muggle and a near-squib. (Or at least Regulus considers him illegitimate. Even though Merope Gaunt and Tom Riddle had married, a marriage while under the effects of a love potion probably wouldn't be considered binding or would be considered very good grounds for dissolution)
The sheer hypocrisy of the whole thing made Regulus's little world crumble. He also suspects Tom was likely secretly very amused at being able to practically make slaves out of the children of those who would have looked down him in school for his ancestry. (If they'd known it. They did know he was returned to a Muggle orphanage over the summers...)
Regulus does not easily lose his temper, raise his voice, or use profanity. He's far more the 'don't get mad, get even' sort. However there are some things that will cross the line with him. An identified danger to his family is the top of that very short list. Even when he's angry, though, he takes after his father in usually remaining outwardly calm and very calculating. (Sirius, instead, seemed to have inherited their mother's temper) In this (rare) mood, he can even be cruel, though he would probably regret that later when he's calmed down. It's unlikely any of his school 'friends' would have ever seen him like this, as Regulus would more than likely excuse himself and leave a room rather than get upset and make some kind of inappropriate scene. It's even possible nothing really seemed enough of a threat to trigger this reaction until Voldemort. Even after he'd calmed down, the resolution remained in place to find a way to make Voldemort regret thinking he could just use the House of Black...
Peter Pettigrew is another topic that can trigger a rare angry Regulus. He hated the Marauders in school for being Sirius's 'replacement family', but at the same time that was always tinged with some hurt as it was Sirius's choice and he could tell his brother genuinely cared about them. He'd overheard some discussion of others among his old acquaintances who were grooming a 'weak link' from the former 'Marauders' to turn spy. It wasn't hard to guess who they meant from some things said, and he has no doubt they'd get Peter to turn, if he hadn't already. It infuriates him to the point where he can't even see any irony in the 'family' that his brother abandoned him for turning around and betraying him. He hates Peter for being a threat to Sirius, who, in his heart, he still considers his brother. But, strangely enough, he also hates Peter for being a threat to the Marauders, who seemed like closer 'family' than his own was, and who Regulus knows probably - with Peter as exception - would have died for each other. That's the kind of family Regulus really only had in his dreams, the kind he wishes his family could be, and to think of Peter throwing it away... well... if Regulus ever catches Pettigrew, he might actually be able to cast a proper Crucio for once in his life.
As he made his plans it started to get more clear to him that he wasn't going to survive. If he did, he would be thought a traitor, and probably tortured to death, possibly along with his parents if the Dark Lord was in a particularly cruel mood. If he went into hiding, then his family would likely be tortured and possibly killed to draw him out. Dying was the only way he could think of to keep them safe. Somewhere during the bargaining stage of this realization, with his plans already mostly formed as far as dealing with the locket, he took the chance to write to Sirius, asking his brother to meet him in secret. He had the last, desperate hope that maybe his brother would think of some way out that he'd missed. As far as Regulus knows, Sirius burned the letter.
He spent a good part of the last day of his life dressed as a muggle (having transfigured his robes for a disguise), in the park he'd asked Sirius to meet him in. It was chilly, but not snowing, and for most of the day he was the only one there... except what seemed to be a large black stray dog that he ignored after an uncomfortable moment of noting that it looked rather like a Grim. He may have commented out loud, and a little bitterly, that that was appropriate. Other than that he didn't speak for most of the day. When the sun went down and the stars started to come out, he gave up on his brother.
He said his goodbyes instead to the star his brother had been named for, once it appeared in the sky, before apparating away. (Hinting that he expected to die soon, but not mentioning the horcrux or his suspicions about Peter since he thought he was essentially talking to himself. Sirius had thought for sure it was a trap, and, tragically, decided to reveal himself just a split second too late. This is what left him with the idea that Regulus had tried to back out and was killed for it - It's not a story I think Voldemort would have tried to spread on his own, given the rest of Regulus's family (sans Sirius) seemed loyal, it would have been better strategy to blame Regulus's disappearance on the Order. And, yes, I know the book has Sirius say that's based on stuff he found out after Regulus died, but... he probably wouldn't have really pieced together all of it until Regulus had been missing long enough to be considered dead.)
Even if he could later learn about Padfoot, he might not make the connection to the dog seen in the park - given the circumstances, he was almost certain that was a Grim.
He went to the cave that same night, concerned his resolve would falter if he waited any longer. He'd gone over a few things with Kreacher before leaving, but didn't tell the house elf most of the details until they'd arrived in the cave. Kreacher protested - for the first and only time in Regulus's memory, at least regarding anything he'd asked of the old elf - but at Regulus's insistence Kreacher did as he was told... including taking the locket and leaving Regulus behind to the inferi.
He (almost) died in early winter (November), not long after his cousin Narcissa let the rest of the family know she was pregnant. He was 18 years old.
Peter Pettigrew turned spy a short time later.
Regulus's father, Orion Black, died that same year under suspicious circumstances, as did his uncle Cygnus Black. Orion never believed the official Death Eater story that the Order - specifically Sirius - had killed Regulus, and after placing numerous additional defensive enchantments on the family's already well-secured home, he left to confront his oldest son about the matter. Not being a fool, he hadn't gone without backup, asking his wife's brother, Cygnus, to accompany him. Unfortunately, Cygnus mentioned the impending visit to a member of his own wife's family (Rosier). Neither one returned from going to see Sirius, and death dates appeared on the family tapestry.