When Peter cries in front of Valentine, she believes it is real weakness. But how is he still manipulating Valentine?

During their conversation, Valentine is taken in by what she believes to be Peter's sincerity. This belief, however, doesn't stop Valentine from bringing up Peter's abuse and murder of animals, specifically, the flayed squirrels. Peter's subsequent outburst, tears, and his vocalization of the fear that he might be a monster draw Valentine back in. He makes her believe that with her help he can change..... avoid becoming the monster. Although Valentine still suspects that Peter has other motives, she is moved by his words. He is able to manipulate her because she "wants" to believe him..... not because she believes him.