Before everyone decides a patron with a gun might have changed things you need to consider one thing. For any of us not trained as First Responders our first response would have been to GET THE HELL OUT OF HERE. Be it to save yourself or your family.
Hindsight is a wonderful thing, as they say it's 20-20. So is second guessing. However placed in the same position as the patrons of that theater, even if you carried a gun, your first response would be to find safety not to make an aggressive move.
Christ, all we would have needed to make things worse for gun ownership would have been a shoot out in a smoke filled dark movie theater by persons with their concealed carry guns and the "perp". The carnage would probably have been worse. Face it, most of us here do not have the skill to respond properly in the CO situation.
It is a sad fact that many of us who own guns and have carry permits do not also take the time to learn how to REALLY use your gun. The right of gun ownership here in the states does not include any skill requirements or eye tests. I liken it to just giving out a drivers license without ever learning to drive because it is your "right".
My opinion is contrary to that of the NRA of which I am a member. But I think to own a gun or hunt you should be required to demonstrate at least the basic skill to safely do so.