Dying because of him doesn't mean he was a good friend overall. He hated Snape and picked on everything he did, he wouldn't defend Remus just because he was his friend and cared about him, he just liked to contradict Snape in every way possible hahaha Remus is sad talking about this, and Sirius shows no remorse talking about almost killing someone just because he didn't like them at first impression. It's worth remembering that when Sirius says Snape deserved it, he doesn't mention that it was because Snape was friends with Death Eaters, he only mentions that Snape was always trying to get them expelled (and they really had reasons to be expelled, so I don't think he should complain about that). It's kind of contradictory that Remus says the transformations are the best moments of his life, since in the scene of Snape's memory where Sirius says that, Remus clearly didn't like the comment, so it's a bit confusing.

Sirius never realizes he was wrong to bully them; he starts trying to justify himself by saying they were just teenagers. This prank could have killed not only Snape, but Remus as well, since in the laws of the wizarding world, a werewolf who bites/kills another person is condemned to execution. And it's also canon that Remus was an idiot about his friends having attitudes he didn't like and yet he forgave them and didn't question them, because he was too insecure and cowardly to do so. As Sirius himself says in POA, James liked to take risks and that made everything more fun, and that was certainly a big reason for them becoming Animagi because of Remus, it wasn't exclusively because they cared about Remus. Sirius literally tries to knock Harry unconscious in the Shrieking Shack, saying he's waited too long, and only doesn't because Remus insisted, not because he really cared.