Peter turning over to the dark side;
Peter's mother was killed by Death Eater's. While this is never proven, Peter doesn't even tell his friends this, he thinks it constantly for almost two years straight, before he go's crawling to Voldemort and offering his support. James and Lily had a baby now, and Sirius had a girlfriend he was always hanging around and Remus was attempting to get a job at Gringotts since Goblin's don't care he's a werewolf... and Peter had no one.
It's gradual, but Peter begins to feel a bit left out, so he convinces himself his years at school were all twisted, and that his 'friends' hadn't really seen him as such. What should he care if his Master wanted some whelp. What was it to him if the purebloods took over, then there wouldn't be anyone to tease him ever again. He wouldn't need those so called friends, who were now too busy for him anyways.
Of course, that's what he told himself, the true act of his betrayal didn't seem to hit home until he saw their bodies.
After the little scene Sirius had made, and Peter's bigger scene that gave him some relative safety for now, he went to Godric's Hollow to see just how much Black had been exaggerating this time.
James and Lily were dead. So was his Master.
He could not return to the life he had, he had ruined that. The idea of hiding out to live with muggles was almost as unbearable as being sent to Azkaban, so he contented himself with staying in this rat form.
It wasn't ideal, but it kept him safe, just the way he liked it.
So I've seen a lot of theories, ranging from even before school or right afterwards Peter was a sniveling coward who ran to Voldemort first chance he got. Then there's the ones that try to defend Peter, say he was tortured into giving up the information. I see this as being a happy median. Peter was scared, and that fear built with time. Then when he did go over to the Dark Side, he couldn't turn back... hope you liked