James' POV
Peter was always cleverer than we gave him credit for.
He may not have managed Outstandings half as often as the rest of us, but he manipulated Sirius like a pro and without us ever realising. He knew all the right words, right bribes, right times to suggest things and he could almost control Sirius's moods as well; prolonging periods of depression or mania, leading him on and baiting him. He directed Sirius's anger towards Severus and his irritating energy onto Remus. He made little comments, little suggestions at just the right times. He plotted with Sirius until my brother in all but blood sent Snape down the whomping willow tunnel.
He planted the doubts about Remus' loyalty.
He was the one who helped persuade Sirius to switch as our secret keeper. He pointed out to the edging-towards-depression animagus that the Black family knew far too many triggers to his psyche, and Remus knew even more, and managed to get Sirius to believe that he would most likely crack under torture or questioning. Made him believe that everyone would suspect him as secret keeper, that he would be putting us in danger. It worked, by controlling Sirius he controlled us all, and I never saw it.
It is so clear from the outside, hindsight that I cannot even share. I watch, helpless, as Sirius challenges him, walking right into his trap.
Peter Pettigrew, little tagalong Peter, he decided the fates of us all.