This is a true and very important point.
In comparison to the general population, hardly anyone carries a gun. That's why these crazed single shooters can estimate that they will likely be unchallenged when they begin their mayham. Even if we doubled the gun carrying segment of the population, that estimation would still be pretty good.
Another important point is that even if there were, say, one citizen gun carrier in the audience, what is he/she going to do when the bullets start flying?
Running like hell to take cover is gonna be very high on the list of options. And properly so. Waiting around to see if you can engage is probably a high risk-low benefit strategy in this case.
It would not surprise me if there was a CCer or two in the audience last night. Who might've been unable or unwilling to take on a guy with a (AR-15?) rifle. Self-defense doesn't mean automatically shooting it out with some goof with a gun. If I'm at a movie with my kids and shots are fired from one direction--I grab those kids and head out in the other direction--even though I have a perfectly good gun on my hip and a reload in my pocket.
So even if there are GG guns in the house it doesn't mean that the shooter loses any bit of the advantage he has once he starts shooting.
Better to figure out how to keep the gun out of the hands of the murderous goofballs if possible, and if allowable under the law.
Hank Good point, As responsible gun owners how many have taken tactical shooting or defence tactics classes? I the enviroment that this was happening in it would be a very high risk low benefit situation. This guy had some type of body armor but was it a quality product or an e-bay special. He like most psycopaths has a very high intellegence level and planed this well. Say someone in the audience had a gun, say a .375 just say'n, and they were able to get a shot off. If this person missed and hit someone running in the path of his round that would be bad, or if he missed and the round killed someone in the theater next door this too would be bad. The shooter was no doubt a coward because he was wearing body armor in front of an unarmed audience, he gave up to the police without incident, he didn't even have the guts to put a round in his own head like most of these psycos do. If he was hit by a .357 round and he had quality BA he would be hurt and probably would have run, if the BA was not quality he would be dead. But that like you said would be unlikely. When someone has planned something like this there isn't much you can do except pray and cover.