Happened in Houston about 3 miles from my office today. Domestic violence, woman screaming for help. Two men come to her aid and one of the rescuers gets shot by the abuser.
I've also read of stories where a woman is screaming for help, bloody and beaten. The police arrive and are her savior. However, when they start to handcuff and arrest the guy, suddenly that same whimpering wife is attacking the policeman with a kitchen knife to "defend her man".
Domestic violence encounters are scary, weird, unpredictable, and dangerous. I don't think I could stand there and watch someone get murdered in cold blood, but I would do everything in my power to avoid having to intervene short of that.