It’s always a possibility that someone does carry a gun in that situation. And then accidentally kills a couple of people while trying to take out the shooter, because he was under adrenaline and in a dark theater in a middle of a panicking crowd. And possibly even runs out of ammo in the process before taking the shooter out. All of which is much more likely than him being an unflappable hero who takes surefire shots in such situation. What do you think that would do for gun laws? If you wonder how well regular people will fair in this situation you can take a look at stats for cop and other enforcement agency shootings, and see how many shots hit out of the number they fired. And those guys do it for a living.
EDIT: All of this is moot point btw, because in this case the shooter was also wearing bullet vest, and other armor. They had it happen in that north Hollywood robbery, it took a SWAT team to put them down.