QUOTE
Plot 3: This is the summer that Peter starts to feel tempted to the dark side, for some reason.
According to the Lexicon, Peter was coerced to the Dark side after graduation. I'm not certain he ever came up with that idea himself.
I quote from
The Lexicon's entry on Peter Pettigrew:
QUOTE
Peter was a person who is easily influenced by those around him, allying himself with whomever is the strongest. Lupin, Sirius, and Potter were the leaders in their year so he naturally followed them. But these were the years of Voldemort's power, and the Potters were for some reason particularly sought after by him. Once out of school, Pettigrew was an easy target for the Dark Lord. He was coerced into becoming a spy for Voldemort. As he put it,
"Sirius, Sirius, what could I have done? The Dark Lord... you have no idea...he has weapons you can't imagine...I was scared, Sirius, I was never brave like you and Remus and James. I never meant it to happen...He-Who-Must-Not-Be-Named forced me...He--he was taking over everywhere! Wh--what was there to be gained by refusing him? You don't understand! He would have killed me, Sirius!" (PA19)
I'd much rather write him as turning bad on his own, but this is the Canon Fodder category and I don't think it would be appropriate.