Namira's quest.
Spoiler Eating people? And be rewarded for it? How on earth did they come up with that idea? Ok if you play a really messed up character, or possibly a Bosmer, but hey... Even the Daedric quests that simply demands an evil acts (like Boethiah's for example) seems more ok - I can see someone being greedy or corrputed enough to go through with that, but to actually eat another human being? It's possible I have my cultural glasses on now, but the only circumstance I can see a "normal" (whatever that means) person do that, is if the alternative is starvation to death, or if the alternative is that someone will kill/torture you. Which isn't the case in this quest, if I got it right (now, I haven't been able to bring myself to go through with it).
I thought it was a great quest, precisely for the reasons you listed: it's one of very few quests that rewards you for being a messed up person disregarding any social or moral norms. It lets your character act up on their twisted, shameful secret desires. Sure, these desires are a major mental disorder and I can't imagine any normal person taking part in that creepy meeting, but who said that our characters have to be completely sane? Alternatively, if you're a werewolf, it may not look so evil after all.
If you don't like what you're asked to do, you can turn around and leave. If you believe that it's evil, inhuman, dangerous and criminal, you can kill everyone involved. It's your choice.
What I find interesting is how many people go through with this quest simply because they want the reward. I never could. Only with one character, and only because it was a cultural thing for her and she was a psychopath to boot. The rest cringed at the very thought of doing what was asked of them.