Honestly I wish we got more of his character in the books because I wanna know what truly got him questioning Voldemort's wrongdoings AFTER he decided to join the death eaters. Because in canon, it says that by 1979 he began to have doubts about his own motives. Since we know that Sirius was clearly the rebel kid who stood against every pureblood ideologies, and Regulus clearly used to uphold every single pureblood traditions unlike Sirius, gives away the fact that he was indeed a blood supremacist. But what was his turning point actually? If we see it from third person's pov, he joined the DE 2 years after Sirius's departure. Means that he didn't do it out of spite to annoy Sirius, but because Regulus and his parents agreed with Voldemort in a lot of things. I wonder why Walburga and Orion didn't join DE, probably because they had an impression to uphold in the pureblood elite society, to control things in a neutral way/ more like fairplay so that if things went south, they'd be able to handle it. So Regulus volunteered instead.
But here's the thing, Regulus was clearly young and he was indoctrinated by his family to believe their ideologies, which is why he joined the DE in the first place, but you don't join Voldemort without knowing about the fact that he had tortured countless amount of muggleborns and set their colonies on fire to obtain the power he has now. News of Voldemort terrorizing the public were all over Daily Prophet. The black family clearly knew all of it and still went with it. But what convinced Regulus to betray Voldemort like that? He clearly knew what he was getting into, but in order to start questioning your own morals after years of indoctrination, there has to be a turning point. It's said that he was not ready to face the truth of being a death eater, does that mean Voldemort was asking for more from the Blacks which convinced Regulus to betray him? We do see that kreacher getting sent in that cave and returning bruised was Regulus's final turning point. Canon seems to indicate that Regulus cared a lot for his family, and he betrayed Voldy to protect them all.
I wonder if Sirius had something to do with all this. Even though as op mentions that Sirius interpreted Regulus's betrayal as something Regulus did out of the love he had for kreacher. But I think there's more to that. Maybe according to him, he and Regulus weren't close as brothers. Which is very clear. But I think to Regulus it was a whole another topic.
I also saw another post where they said that Regulus probably joined Voldemort to collect information about him on how to defeat him, which is why he collected all those articles about Voldy, indicating that harry potter is an unreliable narrator. Which can be true given some moments. But that makes everything so much more complex. If you ask me to give my honest opinion, I'd disagree with it. It seems quite unlikely, given the way his personality was. He probably joined the DE to protect his family for many unknown reasons we will never know. But I don't think Sirius's departure made him think twice about his beliefs. Yes, he came to his realizations, but it's quite clear that he questioned his beliefs only after a long time while being a death eater. For many reasons, I'll talk about it later. But what do y'all think?
p.s- please excuse my grammatical errors ehe I'm not a native speaker.
ngl, if you believe that Regulus's true beliefs are unknown as the only piece of canon information we have on that front is from Sirius, which could be biased because of the animosity between them...I don't think you can say Regulus took revenge on Voldemort for Kreacher because we learn that from Kreacher himself, an elf that literally worships him. So for all we know, Regulus could've been a blood supremacist/voldy fan, could've been not, could've loved his elf, or could've seen them as inferior beings and only betrayed Voldemort simply because he knew Voldemort needed to be stopped.
...im not saying anything, im on the side that he loves his elf, but I was arguing with someone about Regulus as a character and they told me what's mentioned above. That Sirius's dialogues on him aren't reliable for that reason and that Regulus defected when his elf got hurt.
So it's just...idk man.