What's the reason behind violence against women?

Several reasons. When a sub-culture accepts that men should be more powerful than and control women, harming them to maintain control becomes acceptable. If violence was witnessed as a child, a man may believe it is ok. Some men believe that it is part of "being a man." if a man was traumatized by violence as a child, he may not have skills to manage anger or his own behavior. He could be mentally ill.