Here's the thing. Most guys who are physically abusive to women are mentally abusive way before that. These women, while some are indeed idiots for staying, have a hard time leaving for good reasons. A guy who will beat his woman usually alienates her from all her friends first and insults her all the time, taking away what self esteem she has. Having a boyfriend insult you is much more damaging that your average insult. I mean, this is a person that you think cares about you and that you think loves you. If they say something hurtful, you'll probably believe that it is true and try to convince yourself something about how he's telling you that only because he loves you and something something. So here he has taken away your self esteem, taken away your friends and what support you would have if you did leave. He usually alienates you from your family as well, taking away more support. Then he beats the crap out of you. You have no place to go, no one to turn to, and deep down you still want to believe he loves you and that he'll change. You want to be forgiving and you give him another chance. He beats the shit out of you again and again. Maybe if you finally get the strength to leave, he hunts you down and beats you within an inch of your life. The cops don't do shit to help you, so you go back to him because at least when you were with him the beatings weren't as bad as when you made him mad by leaving.
Thank god I've never been in this situation and none of my close friends have either. My aunt, unfortunately, had a horrible experience with a guy that she was living with. He abused her, mentally, physically, sexually, and not just punching you in the face type of stuff, but really perverse, this guy is way fucked up type of stuff. Why did she stay? Cause he threatened to hunt her down and kill her kids. She believed he would have, we all did. I think she could have left if it was just her, but somehow by staying she was protecting her kids, at least for a little while. When he finally moved on to raping and beating her 9 year old daughter (he wanted her to have his baby -- I don't even know if she was physically ABLE to have a baby yet), well, then she got the strength to get them all the hell out of there.
Her and her kids all had to go to therapy and we're quite right for a while. They're fine now, pretty well adjusted and everything. Oh as for the guy, well to get away from him, my aunt had to kill him. It was probably the smartest and bravest thing she ever did.
It's a lot harder than you think. It's easy for us on the outside to say, well he treats you like shit, so just leave him. There's a lot more that goes into it though. That's my two cents anyhow.