M (1931) directed by Fritz Lang and starring Peter Lorre as the fictional child-murderer Hans Beckert is a German crime/thriller film. However, I'm including it as a horror film because it's about a serial killer. A serial killer is always creepy and disturbing, especially one who murders children. In this film, the police are having a difficult time catching the child-murderer. So, a bunch of criminals decide to help out with the manhunt.
I love this film. It's incredibly well done. Peter Lorre is perfect in it because he acts so awkward and creepy. Especially when he whistles the freaky whistling tune, "In the Hall of the Mountain King."