100 people? Jesus. I'm not familiar with shootings like this though, how do they usually get stopped? Do people run and wait for 12?
Play dead.
Dude is mowing people the fukk down indiscriminately so while he's focused on other things, pretend to already be hit and then take a chance and stay or dip thru the closest exit. A dude blasting that many people doesn't exactly have a chopper on the roof for an easy escape. He knew he wasn't walking out alive.