Decent watch at best, probably won't watch again, and can't recommend.
This is an interesting twist on the substitution trope, but it uses two awful people on both sides.
It really feels like a movie made for actors and it is supposed to shine a light on the perception of a person and the reality of a person. Even starting with a down on her luck, kind, nobody shows how she can grow into a monster just like any other person with a taste of power. Maybe it just changes who the victim is.
Once that becomes apparent, it's just sort of "How far are they going to push it?".