tl;dr version: randomneses posts a couple of essays saying that Snape was kind of a jerk and that Lily really wasn't an evil disloyal bitch-harpy for deciding that being called "Mudblood" was the last straw, and Slytherfen went nucking futs all over her LJ.
You know, one of the most interesting things about the HP fandom is the way people will nurse incredibly strong loves/hates for particular characters, and then write pages and pages about how the character they hate was clearly a sociopath, sometimes based on a single incident.
Almost all the people arguing strongly for any particular hypothesis completely eschew any degree of nuance or shade of gray; every action is a character-definining moment, and every character is unambiguously Right and Good or Wrong and Bad.
I think a lot of people see the series as a metaphor for their personal high school experience, and map themselves and that bully who gave them wedgies in gym to various canon characters.
This is most evident in the Slytherfen who hate James, Sirius, and Lily. James and Sirius were sociopaths because they tried to kill Snape by luring him to get killed by Remus, you see, and Lily, well, she smiled – smiled! – when James hung Snape upside down and pantsed him.
These are people who clearly got picked on a lot by jocks, and of course, Lily is a stand-in for that hot girl who was a totally evil bitch-slut because she'd only date guys who weren't them.
It's evident from the books that James and Sirius were jerks as students. They went around hexing and pranking people, they were arrogant and cocky, and they probably did bully Snape.
Where I depart from the Marauder-hating crowd is the assumption that this represented the end of their character development. I can kind of sympathize with that view, since Rowling did a lousy job of showing us character development (and not just for the Marauders). She told us, but never showed us, that James became a better man, that Harry was a true hero, and that Ginny was a strong woman and his soul-mate.
But the fact that someone is a jerk in high school does not mean that they are indelibly stamped as jerks for the rest of their lives.
This is one reason why I give more of a pass to Draco than I do to Snape: Snape was a grown man who had plenty of time to get over being a petty, nasty, insecure bully, and chose not to. To Draco, I can extend the benefit of the doubt, due to the Epilogue, and assume that while he may not have become the chairman of the We Love Muggles Club, he probably did regret the actions of his youth.
Getting back to James and Sirius; it seems really unlikely to me that they truly intended to kill Snape. When James realized what Sirius had done, he went to save him, after all. And to those who think that Sirius, at least, was trying to kill him, I'd remind you of all the mind-bogglingly stupid things teenagers do without really thinking about the consequences.
I imagine the conversation went something like this:
Sirius: James, you'll never guess what I did to Snape!
James: (laughing) What?
Sirius: Well, you know how the prat's been following us around trying to figure out what we're up to on full moons?
James: (snorts) Yeah?
Sirius: Well, I told him if he wants to know so bad, he can go see for himself!
James: Uh, what?
Sirius: I told him how to go through the secret passage to the Shrieking Shack. I just saw the greasy git heading towards the Whomping Willow. (clutches sides) Can you imagine the look on his face?
James: Uh, yeah – right before Remus rips his face off! Are you mental?
Sirius: (stops laughing) Well, umm, I figured he'd... run?
James: You are such an idiot.
As for that oh-so-incriminating smile of Lily's, which Lily-haters take as evidence that she was just another Mean Girl: have you never seen something shocking and embarrassing happen to someone, and your first reaction is to snicker, or even let out an embarrassed laugh, even though you're horrified? I know I have.
I don't think anyone (real or fictional) deserves to be judged according to one bad moment.
That's also why I don't unequivocally condemn Harry for his use of Crucio in Deathly Hallows. It was a terrible moment, but he was a teenager fighting in a war; people do bad things in the heat of the moment.
I do, however, condemn Rowling for treating it as a gosh-cool moment rather than a moment of anger and weakness that should have had consequences.
ETA: I have a small request to make of people who comment without signing in. I allow anonymous comments because I have no reason to f-lock anything here, and I've never had a troll problem. (Knocking on wood...) However, if you do comment anonymously, could you do me a favor and sign your comment with a pen name or something, especially if it's one I will recognize from elsewhere?
I'm not going to delete your comment or anything if you forget to do that (or just don't want to for some reason), but I do like to try to remember folks as best as my porous memory will allow. ;) Thanks!