Peter actually hates quite a lot of people, which would have been surprising to anyone who knows him to find out because he doesn't seem like the type of bloke who hates anyone at all.
Peter hates that too: hates himself for being unable to express his hatred so that it is forced to bottle up inside of him. One day, Peter knows, he will burst, and all of his organs will paint the walls of Hogwarts, and when they find his heart it'll be cracked open and inside will be a list of all the people he hates. His own name will only be second; first will be James Potter. Predictably, Peter hates that too, that even in his own ridiculous metaphors in his own mind James is and always will be above him.
Sometimes Peter looks at James, or at Sirius, or at Remus (third and fourth on the list, respectively), and wonders why, exactly, he loathes them so much. They're decent people, he tells himself. They're his friends.
And then James ruffles his hair in that "older brother" way, or Sirius interrupts him, or Remus laughs even though his joke wasn't at all funny, and then he remembers.
Peter knows that he's pitiful, so it's not their fault, really, for pitying him. But he also remembers a time when he could get an EE on an essay and not be disappointed because the rest of his friends have all gotten O's. He remembers a time when he could look in the mirror and decide that his hair looks good today instead of sighing because it will never even approach Sirius's. He remembers a time when he was content with his imperfection, was happy with his imperfection, because until three light bulbs show up that are brighter than you could ever be you don't notice how dim you were all along.
Dim, that is, metaphorically. Peter isn't stupid. Peter isn't ugly, he's not boring, he's not cowardly or untalented. But when you have the dubious pleasure of being forever measured up against giants, you realize at some point that no matter what you do you'll never even play in the same league. So you stop trying. At least, Peter did, and that was more than three years ago, after he'd gotten his OWL scores back and they'd exceeded expectations but James's had been outstanding.
That was when he'd given up. Giving up consisted of asking for help before he needed it, of doodling during lectures and going to bed with a pile of unfinished homework on his desk. The worst part was that no one even noticed- James commented once or twice, but when Peter snapped at him he back off right away.
So Peter stopped trying (because there would always be someone better than him, if not the Marauders then Lily or Alice or whoever he ended up working with), and all the while he hated, and hated, and hated.
And when the dam bursts, when the floodgates open, when he at last explodes, it isn't Hogwarts that's spattered with his blood and guts. Instead, Peter grabs his cloak and wand, steps into the Floo, and says "Malfoy Manor!" For once in his life, Peter is doing something, taking charge, and it feel good (until it doesn't) (but by then it's already too late, far too late).