It wasn't meant to be taken literally. There's no benefit to torturing, mutilating or defiling Ser Zephirin. He was killed and that's that.
The Dark Knight quests give a pretty good idea as to how dangerous it is for the Warrior of Light to give in to his darker desires anyway so it wouldn't really be of any benefit other than misguided fan appeasement. I wish people would settle for the fact that we've already got Hydaelyn's 'plot armour' and therefore ultimately end up taking down anything in our path. We don't need to go overkill by taking it even further.
