If everyone in that theater was armed and had at least rudimentary firearms safety training... how far do you think the crazy guy would have gotten?
Obviously, nobody can answer a hypothetical question. However, let me ask one in return. How many people would have died in the crossfire if everyone in that theater was armed, only had rudimentary firearms safety training, and all started firing trying to take the crazy guy down? It was dark and smoky. The movie special effects were ongoing. If everyone pulled weapons and started shooting, I'm sure that at least one would be hero would have been misidentified as an accomplice by another would be hero.