There were hundreds of people in that theater and no one used a gun to shoot back? Do mass murderers pick their crowds to eliminate this possibility? There was the one in the Unitarian Church in Tennessee a few years ago. The hero simply tackled the shooter, more or less exactly what Batman (who generally doesn't use firearms) would have done.
Okay, I decided to look it up and accurate or not Wikipedia has a page on "spree killers". Some really fascinating patterns. The killers are overwhelmingly male. Only 2 were female and one of those was in the company of her boyfriend. There's a huge cluster of these kinds of shootings after the year 2000 and I didn't find any instance where one of the victims to be pulled out a gun and shot the shooter.
fwiw, their list is incomplete. It's missing the big Stockton shooting at Cleveland School in 1989 and the Montreal Ecole Polytechnique shootings also in 1989. I am, however, assuming that it's reasonably representative. The other interesting thing is that these things are clearly not strictly an American phenomenon. In fact, the most horrific ones have been in other countries like Norway and Australia.
The vast majority of the shooter/killers were very clearly mentally ill and showed signs of stress well before they did what they did. Whatever one's thoughts about gun control, it should also raise questions about providing services and treatment for the seriously mentally ill. It's not a waste of taxpayer dollars.