The movie clearly didn't have the time, or didn't take the time, or energy, or characterization to fully earn, (much less explain) the twists, it hedges between something really great, and a C list Blumhouse horror. That's why a movie like that is frustrating, but still very good, just an extra 15 minutes, or cut some of the action scenes, and create a more believable foundation for the twists and character's actions, and you had a brilliant movie.
I referenced Jagged Edge in my first post, but it really did seem to have something in common, though hardly enough to be sure, even down to the full body suit, and the twist.