breaking your word to devils, demons and gnolls =/= evil.
And when are you going to see devils and demons surrendering???
As I said earlier, even attempting to negotiate a surrender with diabolically evil opponents is absurd anyway. You have no intention of honoring it, and the only way it would ever happen is if a DM allowed players to metagame it, knowing there is a mechanical chance of it happening even though it makes no sense storywise.