The most telling part of the whole Snape story for me was just Dumbledore saying "You know, sometimes I think we Sort too soon..."
Snape obviously wasn't a nice guy at school and remained not a very nice guy in adulthood. It's also true that everything he did he did for Lily (which, IMO, is completely understandable). But I think the message is that it was getting put in Slytherin which really fucked him up. Had he been in Gryffindor then everything would have been different - he could have been the fifth marauder, or even eventually been with Lily.
Throughout the books almost nobody is a close friend with anyone who isn't in their House. If you were in Slytherin anywhere from Voldemort's schooldays right through to his downfall then I wouldn't have thought that you'd have much of a choice. That's what happens to Snape - why on earth, after all, would you call someone you loved 'mudblood'? Because he's surrounded by this all day, every day. He's got no other common room to go to, and no other friends to talk to. The only people he sees 100% of the time are Voldemort's cronies in slytherin, and that's gonna skew you pretty badly.
Snape should've been in Gryffindor, Pettigrew should've been in Slytherin.