Detroit.....depending on which theater you go into, some of the patrons are probably carrying a gun, probably illegally. Someone whips out a gun and starts shooting, chances are there will be shots fired back. They aren't going to care who gets caught in the crossfire.
I wasn't saying that's what they should do, or even what I would do, but being close to Toledo (nicknamed little Detroit) they have things similar happen, though not to this extent.
but even so - lets say every last person in this theater was armed.....even the 3 month old.....IMO, that was not going to be a deterrent to a person like this, first and foremost. secondly, the guy came in armor. so unless youre a deadeye and nail a headshot, im unsure of what most of the pistols people typically carry are going to do. you run the risk of hitting other people for the chance at delaying him while he reacts to getting nailed in the kevlar? maybe if everyone reacted accordingly, and was cognitive enough to grab their guns and fire on him en masse, but the chances of that are pretty nil too, not to mention the casualties from all the misses... i dont know if thats a great option. let alone the fact that he gassed people on the way in.
im just not seeing the reduced impact of having armed citizens would do here.