Actually, this isn't my only thought on the subject, but it's the only one that I feel like posting, as my remaining thoughts are conflicted, given my longstanding general support of Second Amendment rights (although I don't rise nearly to the belief, widespread though it seems to be, that the Second enshrines the most important of all rights by a towering margin -- in fact, I'm not even convinced that the Second Amendment even says what most people think it says) and my equally longstanding squeamishness when it comes to guns (in short, I've never fired one, never held one, and want no part of either).
But anyway, whenever something like this happens, one meme that pops up a lot is the "If someone in that theater, just one person, was packing a gun, this awful thing might not have happened." Whenever I read that, I don't hear it as an argument for more people toting guns. What I hear is this: "If I had been in that theater with my gun, you can bet that I would have taken that guy down!"
Which is, of course, complete nonsense. From everything I've ever read and learned about these things, this stuff is hard. It's hard even for military sharpshooters to do their job, even with tons of training that Joe Blow Sittin'-in-a-dark-theater-with-a-9mm-in-his-pants almost certainly does not have. I would wager that, of all the people in this country with legal carry-and-conceal permits, the percent of those who would be able to do something with their gun in that situation other than just contribute to the chaos with more bullets flying around is vanishingly small.
One lunatic blasting away is bad enough. Throw in a few pseudo-Rambo's? Ugh.