I'm with Jhamin on this: Suppressive fire is a perfectly legal and valid military tactic. Drow lurking behind illusionary walls is very much similar to an enemy soldier lurking in a foxhole. Sure, it's theoretically possible that the drow might have wanted to surrender, but in reality, a lot of people are often killed while attempting to surrender, simply because the opposition is not aware of the intent behind their sudden appearance, and must react at reflexive speeds.
While the possibility for collateral damage existed, the exalted character was neither aware of, nor involved in, the decision to launch the cloudkill, which was, in and of itself, not an evil act either. Using chemical weapons isn't even outlawed in the D&D timeperiod, since the Geneva Convention hasn't been signed yet. Furthermore, while an exalted character may believe that nothing is beyond redemption, it is still accepted that the middle of a hostile confrontation is not exactly the best time to attempt this redemption....and in any case, redemption can be attained posthumously.