You'll note that I specified "media" as the target of my spite, because this is a common writing trick to add "ethical conflict" to a pretty black-and-white situation. I would simply refuse to take the big bad prisoner, because I'm sick of these lazy fictional quandaries.
What do you suggest as a Good alternative to killing The Dark Lord anyway? Brainwashing them to match your sense of morality isn't a thing good people would do and even if prisons weren't inhumane no D&D bbeg is going to be held in one