Interesting thread and food for thought.
I think it is also social conditioning. Going back to Glenn's comment on people making a coordinated charge for the attacker, I'm sure I recall that being exactly the case in the relatively recent gun attacks in either India or Pakistan (can't for the life of me remember where it was) where the death toll was far lower than might have been because the people near the shooter turned on him and charged rather than turning and running.
Either way, I think for 99% of people in that situation their reaction will be instinctive rather than reasoned.
I must say I have no idea how I would react, only how I hope I would react...