This would be the argument for the NRA's position that we should all pack heat all the time. As they say, an armed society is a polite society. (Which is also the argument for why Iran should have nuclear weapons.) The only flaw in this argument, other than the rash of suicides, accidental shootings, and murders of convenience that would occur if we were all armed, is that in this case, the killer was wearing ballistic armor- a jacket, helmet, neck cover, and leggings. Those kids in a dark theater would have had to be pretty good shots to take him out. I guess the next logical step would be for everyone to wear armor, along with carrying...
Well, the rules for countries are a lot different, and much more Hobbesian. "Iran" (more accurately, the Iranian government) doesn't have the same legitimacy as a democratic country.
In this case, armed people might or might not have made a difference, but
Jeanne Assam certainly did. She was an armed volutenteer security guard, who was Concealed Carrying.
IMHO, it's a mistake to focus on this sort of crazy incident; things like this really would be hard to stop. In the two and a half days since the shooting, there have been about 85 gun homicides in the US. The ones nobody seems to care about. I think we could put a significant dent in those, and there are a hell of a lot more of them than there are from massacres. We ought to look at common sense reforms (interstate transfer, gun show loopholes...) to get cheap weapons off the street. We could do it, but the NRA/GOP don't want us to, and no one (with any power) really gives a crap anyway since it's mostly ghetto kids getting shot.
Nobody ever takes into account the crimes prevented by armed citizens. Usually other crimes such as burglary go up when guns are banned.
If you really want to save ghetto kids from getting shot and/or going to prison, perhaps the drug war would be a better place to start.