What is the worst thing one person could possibly do to another?

I need to know a good public opinion for the story I'm writing. Here's the scenario (loosely written so that no one can directly take the idea): the girl wants to make someone who is normally fairly stony show some emotion, so she has to do something that would really make him hurt emotionally. Since he is more powerful than her, it doesn't make sense to have her him or try to demean him or anything. This would have to be something deeply scarring, like betrayal in its worst form or something like that. Whatever mentally torturous thing you can conjure up would help. Thanks!