Why don't you tell her what your thoughts are? You say that you understand it's your problem and not her's - but she's the one who is in danger of being hurt- so it's definitely a problem she should be aware of.
If I were living with someone who was having fantasies of hurting me, I would want to know so I could make an informed decision about my own safety.
I think it's important that you're showing such insight before the fact, but you should use that insight to do what you can to prevent something you obviously do not want to happen- you don't want to hurt her and you don't want her to be hurt. That's the message I'm getting from what you've written.
If you love her - tell her, 'I don't know why and I hate the fact that this is happening, but I fantasize about hurting you.'
If you can't leave - give her the chance to leave.