There's nothing scarier than a man who has power over you and knows he doesn't have to let go
. In retrospect, the doppleganger acting out to the chagrin of the incel sex pest was the ironic saving grace for Nicki considering it's revealed that she's aware of being taken over. It was clear that the MC was never going to stop brutally SAing her until she died which really twists the knife in.
I really like horror movies where we follow the monster/monster is the main character since it really delves into their twisted psyche of seeing how much they can get away with and their fear/terror feels more cathartic to see since we are rooting against them. 
I know a lot of people got the abusive relationship part of the movie to be really scary and I could see why but it never crossed my mind with how much I wanted the doppleganger to fuck his shit up/more terrified of what the MC would do.
face when they're playing the jenga game