People doing nothing to help is nothing new... see the murder of Kitty Genovese in 1964. http://en.wikipedia.org/wiki/Murder_of_Kitty_Genovese
As to your scenario, I would first need to know who I am? Am I the victim or a bystander?
As for the victim, the first thing is not to become one. As this situation illustrates, there are people out there who will kill you for nothing. What we should glean from that is to keep our mouths shut, our hands off the car horn, our middle fingers in-line with the rest of them and our sneers to ourselves.
If you end up getting pummeled like this, your only real remaining hope is to be able to draw and fire from a retention position. Specifics are impossible, depends on what you know of hand-to-hand, how badly you're hurt by the surprise attack... whatever.
If you're a bystander, well, there are lots of options here too. Depends on what you saw, what you know, what you're carrying, who's in the way... too many variables. We could all come up with a reasonable answer that fits our assumptions but sounds completely unreasonable to people reading it because they're not sharing our assumptions.
Still happily answering to the call-sign Peetza.
The problem, as you so eloquently put it, is choice.
He is no fool who gives what he can not keep to gain what he can not lose.
-Jim Eliott, paraphrasing Philip Henry.