This might be dialogue path dependent. I did this quest with my devotion paladin under patch 2, and there wasn't any fall. I recall there being a lot of dialogue choices weighing up the dreadful situation.
Although, perhaps Ancients are more exacting on this matter? As they focus more on beauty of life over justice and mercy.
We'll see how this pans out patch 4.
There is some argument that not allowing them to go on as abominations and being forced into an eternity of fighting the thirst to steal the live from others, is an act of mercy.
