First, I'd like to express my condolences to the families who have lost loved ones in this senseless tragedy, and offer my sympathy to those who were wounded/hurt.
The Internet is filled with comments relating to this story, and predictably, many contain statements such as "This is why I carry", or "If only someone had been (legally) carrying a firearm". Therefore it is time once again, as it was after the Gabby Giffords shooting, to provide a dose of reality.
In circumstances such as this, and especially in a darkened theater, there is almost complete panic and pandemonium. People begin moving in every direction trying to escape... yes, inexplicably, some in their confused state even move towards the shooter. People push and shove to get through the crowd, some tripping over others that have either stumbled and fallen, or who have dived to the ground/floor to escape flying bullets.
In such a scenario the probability of a citizen with a concealed carry permit being able to get a clear (unobstructed and without being bumped or jostled) shot off at the shooter are almost non-existent. At the same time, the possibility of hitting an innocent person either behind the shooter or suddenly running between you and the shooter are high. So is the possibility that another citizen who cannot see the shooter sees you taking a shot and mistakes you for either the primary or another (possible accomplice) shooter, and thus takes a shot at you. As if your day had not turned bad enough, that could make it worse.
There are many scenarios in which carrying and using a firearm to protect yourself from a life-threatening attack is a good idea... but this is not one of them. Rather than spending precious seconds seeking out one of the few and almost non-existent cases that might allow you to take down the shooter, your time would be better spent concentrating on escape and evasion. If you have properly done your Situational Analysis for the theater, you should know where the closest exits are, and should have made a plan as to how to reach it/them in a panicked crowd situation. Your efforts should primarily be aimed at executing that plan while maintaining vigilance of the shooter's location (or probable location) so as to be moving away from him/her; remember, DISTANCE IS YOUR FRIEND.
With gunshots ringing out, screams and a panicked, frenzied crowd seeking to escape, and especially in a darkened theater, is the person swinging a gun in your direction the shooter? Another possible shooter? An off-duty policeman or citizen concealed carry holder trying to get a bead on the shooter behind you? In that tiny instant... in that scenario... can you be sure? A gun swinging in your direction might be one of those tiny exceptions... by not reacting, I could be shot... but by reacting I might be taking down another CC holder... and, in the process of doing so, now put myself into the same situation from perhaps another CC holder in the same general direction. See the problem?