I will say one thing, when you have something happen like this, what you think you would do, and what you would do are may not be the same.
There was a case I was involved in where a drunk guy was in a dorm, being asked to leave, when he broke the bottle he had and cut a RA (Resident Assistant) across the face. There where a number of people there, many of them who one would expect was tough. Being a college campus of course we where unarmed as after all, a badguy would not use a weapon on campus....
anyways, when this happened, the smallest female ran to the phone to call 911, and the smallest guy tackled and beat the crap out of the guy who assaulted the RA. A number of guys, including one guy who was 6'5 and 300 pounds just stood there going "OMG".
It was kinda sad, the two smallest people did what was right, and the others just stood there.
(saddest thing was there was a school administrator who wanted to suspend the student who tackled the assailant... )
I will be the first to admit that most likely I would not have been the first to jump the guy, but once I saw someone tackle him, I would of tried to go after him till my back gave out. )