The way he viciously kills the girls before going after Casey Cooke, and the way James McAvoy plays the monstrous character in those scenes is absolutely brilliant.
Couple that with the way he snaps out of it and realizes what he has done when Casey says his real name, and we get one of the most heartwrenching horror scenes in recent memory.
We could spend all debating over whether or not Split deserves to be considered a horror movie, as it could also fit a bit more neatly into the thriller category as well.
However, there are a few critical moments in that plant it in the realm of horror, and the best of those moments is the scene in which Kevin Wendell Crumb finally transforms into the “Beast” that he spends the rest of the film teasing.