Two points:
1) Much as it bugs me, I agree with FQ. Several years ago a good friend from my LEO days was attacked from behind, while in uniform and on-duty, in a crowded convenience store during the July 4th weekend. He required reconstructive facial surgery in the aftermath of the attack. During the course of the attack, more than 25 people stood by and watched while this felonious assshole tried to beat my buddy to death. When I asked my friend why he hadn't shot the SOB, his response was, "We carry FMJ ammo. I was afraid a through-and-through might hit a bystander." Personally, I think if they stand by and watch an attack of that severity they kinda would have it coming.....but that's just me. As for myself, I think I'd have a tough time looking in the mirror to shave if I just stood there and did nothing.......again, that's just me.
2) As for how to react when witnessing an assault. Call 911, and let the perpetrator know that you have done so. If there is a weapon involved in the assault, and you are armed, then you might consider a more.....ahem........direct intervention. Use your common sense. If a guy is slapping his gf around you can't shoot him. (Well, you CAN, but you'll wind up in secure, unpleasant surroundings, getting passed around like a joint at a Grateful Dead concert......but I digress.) BOTTOM LINE: Common sense in your actions.