The Russians were told by the terrorists that they would begin killing hostages if their demands were not met. Then they killed two hostages. It is possible that the Russian Govt. felt that they HAD to move or risk a bloodbath. It is also likely that the gas was a last minute grasping at anything that might incapacitate the terrorists before they could kill any more hostages.
Introducing gas into a building is iffy at best since you never know how the dispersal will run. I don't believe that the Russians deliberatly gassed the building in an attempt to kill anyone. I think they did what they believed would be their best chance at getting the hostages out alive.
If thet had gone in without the gas, the terrorists would probably have set off their bombs as they had threateed.

troll