I decided I'm not gonna be a backstabber and instead decided to carve up the boethiah cultists. Boe takes a corpse as a conduit, but then... she is stuck saying that mortal flesh is distasteful and that catching her attention is most unwise. Not dialog, just an idle bark.

Is the quest bugged? I heard you could start the quest by just skipping straight to "slaughter all the cultists" if you're not keen on being a backstabber.

You're not gonna like the way this ends. If you want the Ebony Mail you're gonna have to kill someone you know even if you slaughter the cultists first.

Klling all the cultists only nets Boethiah's approval. So far, I haven't found a way to "fail" the quest.