That's just edgy consequentialism, if both have a 100% chance to succeed then Pacify is blatanly the best option because it means the spirit can be killed painlessly and in a dignified manner as opposed to being left to die in agony.Look at it this way. Suppose Ling Qi could have succeeded in either choice; trying to pacify the spirit or trying to put down the spirit. Based on the new information that it's hopelessly corrupted and will have to be put down by someone to end its pain, then Ending it would have been the right choice of the two.
So if in a world where either choice could have succeeded, Ending it would have been right, then in a world where either choice would fail than you might at least have failed trying to do the more correct of the two options.