the purpose of this quest is to get rid of useless companions and get an ebony mail in return. lorewise, killing your companion doesnt make sense. and as a mage, lorewise, u'd heal ur companion when they were injured. succumbing to a cult and sacrificing them is an easy-out for things.
well my mage did it with pleasure, and used the body for death thrall afterwards
other people are merely a tool for her!
where is TES lore on not killing followers? wtf
u still have to sacrifice a follower to make boetiah appear.
well he appears (100% sure), and is displeased
but he still demands the sacrifice