I'm very sorry to hear about that.
Your Aunt seems to be like a lot of 'battered women' out there in the world. Convinced that they are the blame of such abuse and feeling that the man that they are with is good at heart.
It is natural that you feel the way that you do. However, violence is not the first choice that you should make to deal with violence. It should be the last choice. You cannot inspire a man who is violent to stop with more violence. He will retaliate with every ounce of his being against those who he is closest to. Namely... your Aunt.
There is only one answer to this problem. Your Aunt needs to make the decision. Until that happens... you should respect her decision. Even if it is ludicrous. The only person that can change her circumstance is her. She has to want to see the problem and then take personal responsibility to change it. However, the moment she asks for help... do everything in your power to protect her. Do not 'jump the gun'.