M (1931)

“A psychotic child murderer stalks a city, and despite an exhaustive investigation fuelled by public hysteria and outcry, the police have been unable to find him. But the police crackdown does have one side-affect, it makes it nearly impossible for the organized criminal underground to operate. So they decide that the only way to get the police off their backs is to catch the murderer themselves. Besides, he is giving them a bad name.” — IMDb.com