5. Curb Stomp - American History X
American History X is a towering, monumental drama bolstered by an excellent Edward Norton performance, but is undoubtedly best remembered for the harrowing early sequence which sends Norton's Derek Vinyard to jail in the first place.
Derek attacks some thieves who attempt to steal his dead father's truck, shooting one dead and forcing the other to bite the curb at gunpoint. After shouting, "Say goodnight!", Derek curb stomps the man, killing him instantly in one of cinema's all-time most discomforting murder scenes.
The racially-motivated nature of the crime would be enough to make it disgusting, but the detached brutality with which director Tony Kaye shoots the scene, refusing to flinch away, and the sound of the man's teeth biting on the curb, make it truly hard to stomach.
If you can watch this without contorting your face into a pretzel, you might want to talk to somebody.