That wasn’t a hostage situation that was plain ethnic cleansing. Russia has thrown pretty much the whole of the male population in the occupied territories in the meat grinder by now, btw.
What OP is referring to is the Dubrovka crisis where Chechens took tons of hostages in a theatre and Russia pumped the whole thing full of gas, probably some fast-acting opioid. Which isn’t that bad of a way to diffuse a situation they just failed to inform EMTs on what they’ve used or at least how patients should be treated which meant a lot more respiratory failures than was necessary. Still a better outcome than storming the thing and the Chechens blowing everything up.