This situation made me think about what might have been had I been there, armed, and been fortunate enough to avoid being a casualty early on in the attack.
I usually carry my DW CBOB 10mm auto with 200 XTP @1200fps. If I was still healthy after the attack started, then I would have to assess the shooter. From reports it was obvious he was wearing body armor, a ballistic helmet, groin protection, leg protection, arm protection and throat protection; as well as a gas mask. Assuming the shooter was wearing at least Level IIIA armor, this would leave a head shot at the solution to stopping the deadly threat. Shooting to center of mass would only draw unwanted attention from a shooter with a AR-15, likely making me a priority target and get me killed.
Negatives for armed response success:
1) If the canisters releases tear gas, that would likely reduce my ability to see and breath, reducing the ability to shoot accurately and quickly.
2) The stress of being in a deadly situation would likely reduce my ability to shoot accurately.
3) The panicked theater patrons' efforts to escape would likely interfere with my ability to have a clear lane to shoot, and I would likely be physically jostled or worse while attempting to get a sight picture.
My conclusion is that had I been there, survived the initial attack, tried to engage the shooter, that I'd most likely have been shot while attempting to make a head shot. Only if I had somehow found myself in a position where I was close to the shooter, uninjured, unhampered by fleeing patrons and unseen by the shooter, might I have been able to make a head shot or if from behind a neck shot.
All in all a very low probability of a good/survivable outcome for an armed response in my guesstimation. (Ya a made up word.)
This shooter planned and setup as many factors in his favor for his survival. The only trade-off that might have been to the shooter's detriment was the gas mask, as it might have limited his vision if an armed response had occurred.