M by Fritz Lang (Germany, 1931) Phenomenology of Evil: a serial killer and his social group.

In 1931 an unknown murderer of little girls (Peter Lorre) is terrorizing the city of Berlin. We see him entice a new victim, the little Elsie Beckmann, who is coming home from school: whistling a tune by Grieg, he buys her a balloon from a blind beggar. When her corpse is discovered, the police undertake a major mobilization aimed at seeking the serial killer in the criminal underworld; meanwhile, the ever more terrified population starts to see the dangerous murderer in everyone. Since the roundups and incursions into the seediest parts of town disturb the gangsters' activities, the leaders of organized crime, headed by Schränker (Gustav Gründgens), take it upon themselves also to hunt down the solitary child-killer, engaging the community of beggars. Every corner of the city is catalogued and sifted by the dual activity of the police and the gangsters. The 'monster', a former psychiatric patient, mild and harmless in manner, is finally tracked down via two parallel routes: the clue of a cigarette packet enables Inspector Lohmann (Otto Wernicke) to track down the serial killer's address, while the blind beggar recognizes the whistled tune. The murderer is identified by a 'slap' from a young criminal which leaves an M marked in chalk on a shoulder of the man's overcoat. Thus he is caught and undergoes a kind of trial at the hands of the gangsters who tie him up in order to lynch him, but they are interrupted by the arrival of the forces of law and order.