Shooter has huge advantage
In the situation described, the shooter has such a tremendous advantage over any bystander armed with a handgun that it seems very unlikely that he could be stopped. Just consider the list of shooter advantages:
1. The shooter has a plan and the advantage of surprise - the movie audience is relaxed and not expecting this to happen.
2. The shooter is wearing body armor.
3. The shooter discharged tear gas to start, but he is wearing a mask to protect himself.
4. The shooter has a high capacity rifle and backup guns, while the defenders have one or two pistols.
5. Hundreds of people are milling around in panic, and the shooter can achieve his goals by shooting into the crowd and hitting any of them. The defender must avoid hitting innocent people and focus his fire on one individual target.
6. Even if the defender knows there is a shooter active, he may not be able to see him, given the darkness of the theater and the hundreds of people running everywhere.
7. The shooter has nobody with him to protect, and is probably mentally prepared to die, while the defender may be concerned with the safety of people with him, and is also concerned with his own preservation.
It seems to me that the shooter spent a long time planning this and selecting his equipment carefully, such that he was virtually unstoppable.