If I were the Ruskie planners, I would pump a large volume of carbon dioxide into the theater and keep the level high, causing drowsiness. Then, I would pump a large volume of ____ ___ into the same air system and storm the place to kill the terrorists and save the hostages whose repiratory system had been severely compromised. There are several gas agents (non-nerve type) that will induce unconsciousness very quickly, especially if there is a high concentration of carbon dioxide in the blood gas mix.
a large volume of carbon dioxide into the theater and keep the level high, causing drowsiness high CO2 levels would produce a feeling of suffocation, with attendant panic- when we hold our breath, it's the build up of CO2 in the blood that generates the urge to breathe.
On the other hand, a gas like nitrogen could be used to displace the oxygen in the air, leading to hypoxia and unconciousness without the subject's awareness. The problem there is if the subjects stop breating entirely, you would only have a few minutes before death/brain damage ensued.