I've given that some thought. In a crowded theater, as pandamonium erupted in the dark, with teargas spreading in the air, the tactical opportunity to get effective shots at the perp would not have been favorable unless there were several folks or at least one in a good position early in the event.
In short, I would have been sitting in the middle of the theater. By the time I realized what was going on, I expect bodies would have been going in all directions. I suspect that most of the folks that escaped unharmed didn't get a good look at the perp at all.
Still, it would have slowed down the carnage with some return fire and given people a chance instead of either being shot at the exits or waiting to be executed huddled between the seats.