I will be packing very shortly. I'd get away if given the chance.
However how would I be able to live with myself if I had the ability to do something and I didn't and a little kid was killed. I'd never be able to look at myself again.
Do what? Rush at him while he has an AK and two other guns? No one could have done anything, unless a group of people decided to bum rush him, which would be certain suicide for many. Even if a group was somehow able to coordinate that, it would have been incredibly difficult in a movie theater because you can't rush anyone because of how the theater is divided into rows, meaning you can't all out rush him without being in a single file line. Doing so would probably just have raised the death count.
I would have laid my *** flat on the floor and not moved an inch. Cowardly, yes. Smart, hell yes. The guy was probably gunning down people that moved and probably just shot random bullets into the crowd (which probably attributed to most of the injuries).
Unless you have cat like reflexes you probably wouldn't have been able to make the jump, and run away to safety without being shot (even without considering possible injuries from the fall alone).