Of course, it's hard to know how one would react in a given situation but...
It's 3am, I've been in the bar for a good piece of the evening and not for the first time. I've come to know the barmaid a bit and she's nice and she's cute. I don't know squat about her ex-boyfriend and the trouble she's had.
The bar closes and I'm talking to the girl in the parking lot. Some guy rolls up and starts abusing the girl.
Yeah, I'll get in his grill. I'm definitely the kind of guy who would come to the defense of someone I've been friendly with and who has been nice to me. I'm not a fighter by nature, but I also won't stand by and watch a friend (or even an innocent acquaintance) be abused. Put a few beers in me and I'm even more likely to square off.
This happens all over the place every day. Most of the time there's a bunch of shouting. Sometimes there's some shoving and maybe a few punches thrown. Every once in a while, someone gets really dumb and pulls their piece and starts shooting. Even more seldom, someone gets shot and dies.
It's sad but it happens. That doesn't mean a guy shouldn't stand up to an asshole who's picking on a woman. Seeing a story like this depresses me, but if I found myself in a similar situation this weekend, I wouldn't run away.