“Harry…I as good as killed them,” he croaked. “I persuaded Lily and James to change to Peter at the last moment, persuaded them to use him a Secret-Keeper instead of me…I’m to blame, I know it…the night they died, I’d arranged to check on Peter, make sure he was still safe, but when I arrived at his hiding place, he’d gone. Yet there was no sign of struggle. It didn’t feel right. I was scared. I set out for your parents’ house straight away. And when I saw their house, destroyed, and their bodies - I realised what Peter must have done. What I’d done.”
Harry Potter and the Prisoner of Azkaban
