My father in law has done horrible things to my mother in law in the past. She wants to leave him bu

My father in law has done horrible things to my mother in law in the past. She wants to leave him but he tells her that if she leaves that he will kill her and all of her family. We fell in love and we want to be together. I know he would do it because he is that crazy. What can be done about this? The things he has done to her in the past is illegal. What can be done about all of this? PLEASE HELP!!!!

Tell the cops