The main motif of HP is choices. Voldemort chose to become what he became in the same way Harry chose not to despite the many similarities between them (funny enough, this is much better explained in the fifth movie).
Both James and Snape were assholes (Sirius too, tbh) at some point. James was spoiled and rude, which eventually made him a bully. Alas, James changed when he was still in Hogwarts. How much the episode of Sirius almost killing Snape influenced on it? We might never know. The thing is, he decided to change and become the good James everybody remembers.
Snape is a different case. When they're both presented together before Hogwarts, Snape seemed to be a much nicer person than James. Looks like his friendship with Lily kept him with the feet on the ground. If we take Slytherin house as a metaphor for embracing evil (which it is), then Sirius was right when he said Snape always had a fascination for the Dark Arts and he chose to embrace them (while Sirius rejected them by being sorted to Gryffindor, unlike everybody in his family). James being a bully was probably just an accelerator and a excuse, but not a cause. Snape decided to join a cult that was killing people because they were different and he only stopped when he realised Lily could have been personally targeted. He didn't care the same could have happened to Alice Longbottom or any other woman. Also, his alleged love for Lily was equally selfish because he didn't mind either that James or Harry were hurt on the process. He was not remotely a good person. He never cared about anyone but his own childhood crush being fulfilled. He's not a tragic hero or anything remotely similar to it. People tend to side with him because he was a victim of bullying, but victims can also be cruel assholes.
If any one is an unrepentant bastard, it's Sirius. He's the one who tips off Snape that Remus is going to the Shrieking Shack every month and he continues to be an absolute dick to the man years later, even after he's joined the Order.
Sirius had nothing to repent for. We don't know w hat happened in that incident. He could have, indeed, planned it all to get Snape killed or simply got fed up and yell at him something like "if you want to know what we do why don't you just go and find out?" and never realised Snape would actually do so. Honestly, Sirius seems to be too smart to actually plan to get Snape killed knowing Remus would be the one doing the killing.
Now, if him being a dick is his behavior in Order of the Phoenix, this happened after Snape could have gotten James (Sectusempra in the memory, which was before the SS incident), Remus, and himself (Azkaban) killed. Snape tried to get Sirius and Remus kissed by a Dementor, later made Remus lose his only job because he was mad at him being both innocent, and was overall quite abusive towards Harry for the only crime of looking like James. Sirius was right when he said he didn't trust Snape: Snape told Voldemort about the prophecy, which made Voldemort target the Potters and Snape only wanted to save Lily, not James or Harry. I think Sirius had every right to be a dick at him.
