You would have to be built like REDVAN to take on that guy.
I like to think I'm capable of doing the right thing and have proven in the past to work well in pressure-filled or emergency situations. Notwithstanding all that, I'm almost 100% certain I wouldn't have intervened in that situation. Like most, I would have either fled in terror or stood frozen in fear.
Mikey's comment about others quickly following if only someone would "man up" reminded me of this:
http://en.wikipedia.org/wiki/Jonathan_Burton
As Res pointed out above, the victim stood no chance. This was the best possible outcome for all others involved. I don't think things would have been any better had the passengers got together to stomp the crap out of the killer. Two dead bodies probably isn't any better than one dead body.