as far as i can recall i have never been witness to a man attacking a woman to the point i needed to step in.* and i am glad i haven't as i'm not sure if my instinct would be helpful.
chances are that these people are a couple or know each other and chances are this is neither the first nor last time such activity will occur. stepping in might make it worse; e.g. they go home and he attacks her because it was her fault that someone stepped in or because he could have gotten in trouble, that sort of thing.
again, that is not to say i wouldn't insinuate myself. i would. but it is not some chivalrous moment of fortitude and justice. it is more likely a sad necessity that momentarily delays the physical and verbal blows rained upon an individual.
i am not for capital punishment but serial abusers should be disappeared.** there is no use for them.
* and by that i simply mean an actual abuse/attack situation. and it needn't be man v. woman, it could be X v. man or v. child or v. animal.
** yes, dropped from miles up and hundreds of miles from the coast into the ocean. that disappeared.