Why are some fathers abusive towards their spouse and children and why not some mothers get rid of them?

Hi I am an only child that has been abused and insulted by my biological father when I was a child. He's gone, thank god but now my best friend also has an abusive father. He beats, insults, throws things, curses and get angry over very small things such as forgetting to take out the trash or something. And sometimes he'll just be an *** and hit her and her mother for no reason at all! Whats worse, her mother won't do a thing to protect herself and her daughter. When ever he hits them, the mother blames it on my friend. I wanna know why some fathers are like this and why some moms do NOTHING. I know not all men are like this, but the mass majority of them are and it sickens me. Seeing her in pain reminds me of what my father did to me and I can't do anything to protect her. So please answer me why they are like this towards their own flesh and blood?