I love how JKR was supposed to show us all this random crap from the past generation(as in, before Harry's time), so that we'd have proof of their actions instead of listening to hearsay. "We never actually SEE Mulciber bullying anyone, we never actually SEE Lily acting kindhearted" -- so, with all the extra page-space these walks through 1970s Hogwarts would've taken, when exactly was Harry supposed to get around to his own story? You know, the one actually being told in these Harry Potter books?
I can buy that Mulciber was a bully from the fact that he became an animal executioner for the Ministry, and an active Death Eater. I can buy that Lily was fairly selfless from the fact that she died for her son (these guys trivialize that 'cause it's easy to be an armchair vigilante, but realistically, choosing death when the option to, well, not is selfless). I can buy that James, prick he might havebeen sometimes, was also the kind of guy who'd do anything for people he cares about -- from the fact that he spent half his Hogwarts career working toward an accomplishment he'd never be able to tell more than three people. (They can whine about "glory" and "excitement", but there's no glory if no one knows, and five years is an insane amount of time to spend for excitement they'd only be able to have for another two years; especially for kids). I don't need the Marauder-Era-Life-Story Encyclopedia to just GET THIS.
I'm not getting the logic! Nah, let's not assume Snape might've been a little biased in his opinion. Clearly, every other character in the series whose ever spoken about this era, including Lily and some of the Marauders themselves, are liars OMG.