If the gas in the theater was truly tear gas, you would be in zero condition to do anything about it. A well armed, gas-mask equipped, body-armor wearing assailant against even a theater full of CCW permit holders suffering from tear gas is going to be able to kill with almost total impunity.
Yeah, this is certainly true. The dirtbag really did plan this one well.
Though I will still say that the sign banning firearms served no useful purpose, nor would it ever.
In basic training we had to run into an enclosed space with teargas in it, run in circles, stop and recite our army serial number, and run out. This was CN, not CS which is much more powerful.
If a person has a fighting instinct, they can overcome a lot of obstacles. There is absolutely no excuse for disarming law-abiding citizens.