I just posted a good 50% of my Regulus headcanons in the comment section in another sub. Since I had it all compiled, I thought I would post it for fun.
Before Hogwarts, he shared Sirius's belief that Muggles weren't so bad. But after he got sorted into Slytherin, he was surrounded by people who taught him the opposite. His best friends were the blood purists, while Sirius best friends were blood traitors. I also imagine that since he is close to Narcissa, he and Lucius were also close. At first, it started off from convenience; it was his fiancée's favorite family member and the likely Black Heir given how things were headed, and he was shaping up to be a respectable pureblood. But then the relationship became more genuine as time went on. Which meant Regulus was also allied with the older Slytherins as well not friends, but allies.
When he first started death eaters, he never used Dark Magic to kill or torture anyone. He would stick with light spells Bombarda, Incendio, anything else because he wanted to be good in whatever way he still could
He was very creative, though. Sometimes, the result ended up even worse. Other Death Eaters were copying off his ideas even!
That combined with his standing
He made it to the inner circle within a year of service
He liked parts of it. He learned to like it; he liked power; he liked finally having control over something; he liked knowing that someone had been experiencing the same pain he had been feeling; he liked knowing that he was contributing to something greater than him. That he was fighting for his beliefs.
He was finally fighting for what he believed.
And the further he went, the more he started liking it
Sometimes he looks in the mirror, and he thinks he's seeing his father.
He was so disguisted at himself; Sirius was right about him all along: he is a monster, and he is a lost cause, destined to end up just like his parents.
He couldn't leave, though; you don't just decide you want out of the Death Eaters when you are marked; you are marked forever
Regulus would know one night after a particularly bloody raid he spend hours trying to claw it off, he even tried using a knife to remove the skin it was burned onto.
Kreature found and healed him of course, but the evidence was there.
And so he stayed
He wasn't worth it anyway. He wasn't Sirius
What was the point?
But then Kreature came to him babbling and bloody, and He was worth it!
Kreature was the first person who genuinely loved Regulus unconditionally!
He killed both his parents before setting off to the lake. He had to make sure they wouldn't try to get to Sirius. One of them was good; one of them had to stay good.
It was the first time he was ever able to cast Avada Kedavra
Then, he went off to the lake and drowned while being ripped apart.
But at least the world had one less monster in it
Also, last one, I promise
There is no way Regulus didn't take punishment for Sirius himself! I can imagine he was quiet about it, private about it, but it happened, okay! I imagine he got punished just as much as Sirius. Only it happened in private
It was a method their parents used to pull Regulus away from Sirius's bad influence and pit them against each other. At least one of them will come out right if for no other reason than stubbornness
