Tear gas (CS, etc) is illegal (violation of the 1993 Chemical Weapons Convention, signed by George Bush Sr.) in warfare. Nevertheless, the alternative would be worse, so performing such an illegal act might be the most moral course of action.
That's too bad, the stuff works great on "compounds" be they churches or mosques. Guess we'll have to save this good stuff for the war on civilian disarmament when it comes back home to roost.