Has the dark turn already happened? Would it be possible to know what it is about?
So far from what I've read of two characters (Emily and Nicole; I think those were their names), I'm convinced that if you can finish the game by killing, or mutilating them in a horrible way, or making the rest of their lives suffer and regret, I might be interested in seeing that (Not literally seeing the mutilation or how they die, but knowing about it).
On the other hand, any unavoidable sex scene of them with the MC would be incredibly annoying... From what I've read, if I play, they are totally off my list of LIs, so they are off of any possible harem ending (which I don't know if there is) or, rather, of any ending (except for the horribly suffering part, I include that for them in whatever ending I wanted to see).