Wasn't Andromeda his first cousin or some such? And didn't she get banished after Sirius had converted to the enemy's religion? I'd always thought Sirius just began his rebellious phase early and then himself bowed to peer pressure. James was said to have quite a magnetic personality, after all.

 

It was suggested that Walpurga was pretty damn crazy, so it might just be that he went full-on "screw purebloods" mode largely to oppose his mother. I would have to mine for quotes, but I'm pretty sure it was at least heavily implied that Sirius stance was anti-pureblood even before him going to Hogwarts.

 

Ah. Then Andromeda's banishment leading to the beginning of his rebelliousness makes a lot more sense.

Snape's memories of his first Hogwarts Express ride, when he's sitting with James and Sirius at first seem to imply it. I don't have the quote, but I'm reasonably sure that was where it's mentioned.

 

Honestly to me, the more interesting option isn't that for some reason he was already rebelling against everything that he'd been tought because of some vague feelings of rebellion, but that instead the Sorting Hat didn't sort him based on his politics, but instead of his character. Harry didn't almost end up in Slytherin because he was secretly a dick but because it felt that it suited him because of his personality and that it would encourage him to be more ambitious.

So to me, I really like the mental image of Sirius expecting to be put into Slytherin, but when the Sorting Hat really looks at him, it tells him that it wants to put him into Gryffindore and seizes the opportunity because that's just the kind of person that he is.

 

Sirius in isolation is a really interesting character. He's a man child, with good reason, who's spent more than a decade in hell and escapes to find not only his whole family dead but also with almost no one he cares about still alive. TBH he reads a lot like classical indie!Harry only with a tragic ending.

Could Lily have saved Snape? Eh, probably, I mean in the books she's basically perfect, she probably could have figured something out if she really wanted to. Would she have ever done so though? She's basically a Jewish girl who's best friend voluntarily joined the Hitler Youth.