They're also not that happily compliant.
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When you find everyone's favourite Necrophiliac sitting on his throne with a couple of his spectral hookers waiting on him, there's a soul gem on a pedestal nearby. That's the control mechanism he keeps them under his sway with. Take away the soul gem and they react pretty much as one would expect a bunch of violent, wrathful spirits with one hell of a grudge against him would react.
They're still hostile toward you afterwards, and you'll probably have to put the spirits out of their misery to get out of the cave, but I always make a point of doing that when I complete the quest. I figure the spirits deserve their chance to vent.
EDIT: Also, while that quest was gross as hell, that wasn't the most disturbing quest in Skyrim for me. Not by a long shot. That dubious honour goes to Frostflow Abyss, better known as "That creepy ****ing quest in the lighthouse near Dawnstar".
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Descending down through the caves of the Falmer, finding the slaughtered family with the diaries detailing what happened to them. By the time I found the daughter whose suicide note described watching her father get weaker and weaker, and then ending with "I don't know where he kept this dagger but I think I know why he gave it to me" before the page is obscured by her own blood.... brr. That's the point where I lost any and all sympathy for the Falmer for what the Dwemer did to them. Killing them stopped being a chore at that point - hunting them down and killing them at every chance just became the right thing to do.