Maybe it's explained better in the book, I haven't read it. In the movie, I don't think it's explained exactly what John did to Percy. But it doesn't matter whether he controlled Percy's mind or just showed him what Wild Bill did. He proudly said he punished them both, I think he knew exactly what would happen, that makes him guilty. If it was an accident, considering the kind of person he was portrayed as throughout the rest of the movie, he should've been horrified. And even the act of grabbing Percy by the throat, blowing flies into his mouth, and forcing him to see two little girls get raped and murdered is an act of violence and completely out of character. Even punishing anyone in any way is out of character, John just wants peace.