You seem to know what you're talking about so I have to bow to your expertise on the matter. But from what I understand there was a lot of confusion in the audience about what was real and what wasn't and then add the fact that people were panicing and trampling over each other trying to get out. Some were even injured from getting stepped on as they hunkered down between seats. Then there was the smoke factor that was burning peoples eyes and throat and making it difficult to see who was who or what. The whole thing transpired over a period of 30 minutes.
I think a lot of this after the fact talk about what someone woulda coulda shoulda done if someone only had a gun belongs more in the movies than in a real life situation like that. So without knowing Holmes real motives for the rampage, I think it be difficult to say how he would have reacted if someone did have a gun and started shooting back. He could have paniced and run out or he could have gone after the person shooting at him, who knows really.
I live in Utah and back in 2007 a similar situation occured in Trolley Square which is a two level shopping mall and some kid went on a shooting spree trying to take out everyone he saw. Luckily someone else was carrying a gun but he just happened to be an off duty police officer from another city so he knew better than most how to respond and I have no doubt he saved a lot of lives. But I just don't see the average Joe that carries a concealed weapon responding in the same way at least not without training or prior experience like serving in a war zone. But ugh, this kind of thing just seems to be happening more and more and more.
Other than the person sitting in front of Pee Wee Herman, I've never heard of anybody getting shot at in a theater.
I certainly haven't had the experience and don't want it. Everyone posting here, as far as I know, is, like me, trying to determine how to stop someone like Holmes once he has started. Obviously, getting the hell out without getting shot is everyone's ultimate goal, but you just can't force a theater full of people out of two exits that quickly.
The question is: If you cannot leave, how do you eliminate the threat? "Depend on others" is not a good solution. Given the option of defending yourself or not, I'm guessing you'd want to defend yourself and halt the threat. If someone is shooting a gun at you your options for self-defense are limited. I sincerely hope you are never in that situation, but I'm thinking, with few real options available, you'd probably be willing to shoot someone who was going to shoot you. If there is a better option where me and mine get to live, I'd take it in a second. Wouldn't everyone?
Holmes was smart to pop smoke in the theater. It added to the confusion, as did the way he was dressed. People just don't think something like mass murder would happen in a theater in Aurora on a Friday at midnight. That was to Holmes' advantage as well. Yes, even if there were people who were carrying concealed guns in the theater, it probably would have taken a few beats before they realized that Holmes was no theater promotion.
It's always good to plan ahead and think how you would react in any given environment. There are no safe places in America anymore, even at home. People should realize that. As an example, I never leave my house without looking at my surroundings and thinking how I would react to a threat.
Everyone here, I think is talking their way through what their options would be in a situation like the OP. The options are rather slim. All in all, I think most would prefer to have a way to defend themselves. The best thing you can do is prepare yourself.