I always thought we'd be friends for life, but I guess I was wrong.
You see, when more than three people become friends, be it five, or even four people, someone becomes an outcast. And sometimes, that person does not want to be an outcast. But because there are more than three friends, in this case four, people begin to form 'clicks' together, resulting in the out casting of one person, Peter Pettigrew.
Sure, James, Remus, and Sirius talked to him and treated him like a friend, but when they did, he was a last resort. They only called upon him most of the time because he was the only person within talking distance, or because his 'friends' thought that Peter liked listening to them rant on, or that Peter had no feelings and therefore needed to talk to no one, or even that Peter was stupid, was weak, that he was there just so they could boss him around and make him their puppet! And oh how Peter hated it. So he did the only thing he could do.
He allowed himself to be wooed by the powers of the 'other side', seduced by the rising number of opportunities of a new chance around people who could understand, with a master who could see who Peter really was, and who encouraged the new Peter. Here amongst Lord Voldemort's Death Eaters, Peter was powerful, was dangerous. Here, Wormtail could finally get his revenge on James Potter, Sirius Black, and Remus Lupin, hurting everyone else who betrayed him in the process of elimination.
Wormtail knew that his plan was without flaw. He knew James Potter trusted him with his and his wife's lives, and oh how he enjoyed every minute of planning their downfall, what with being their secret keeper and all.
And now, Peter Pettigrew knew revenge was truly sweet.