My girlfriend has just admitted to me that her dad hits her... I really want kill that fucking piece of shit!!! And she's thinks she deserves it! I wanna get her out of there right now, I wanna kill him...
I'm so sorry you're in this situation hun, it really can't be easy for you and i'm not surprised you're feeling the way you are.
Its tought that she won't leave, but at the end of the day you can't force her. I think the best thing to do is to talk to her, let her know that you want her to leave, that she doesn't deserve it, that you will always be there for her, that you will never do that to her.
You could go the legal route, and as much as i think this is right i think you might also risk her resenting you for getting her dad in trouble.
Offer her your support, your love and your gentleness. I hope something gets sorted soon whatever the solution is hun.