Peter Pettigrew spent his entire school years in the shadow of his more popular, powerful friends. Whether or not this is the reason behind his turning to the dark side is up for debate, but it's not a stretch of the imagination.
In the apostolic allusion (see James's page) Peter plays the role of Judas, the former friend turned betrayer.
In the fourth book, we learn Peter returns to the Dark Lord as his most loyal servant and brings about his return to power, though from his behaviour in it all, he seems less than enthusiastic about his duties, but too scared to run away.
~ James Potter | Sirius Black | Remus Lupin | Peter Pettigrew ~