If a man beats his wife and children, and gets sent to jail, he only stays there for a while, and then goes back home, where he probably beats them much worse than he did before. However, if a woman kills her husband, to stop the beating from happening, then she gets thrown in jail for a sentence of 15 to life. Of course, I'm not saying it's right, but the man usually has it coming, cuz he makes her believe that there's no other way out, or that he'll kill her first.
A good example is the June Briand case of 1987. She killed her husband, because he found out that she was going to leave him, and she was convinced that he was going to kill her. She was convicted, and sentenced to jail for 15 to life, but only served 10, cuz she proved thet she was no threat to society. For more info on this case, there's a book by Robert Davidson, called Fighting Back. It's pretty-much just a biography on June, telling what led up to the killing, and what happened after.