"Masterpiece" is the word most film buffs and historians probably would use to describe this 1931 Fritz Lange film, based loosely on the events of a real-life serial killer a few years prior to the film. In this movie, the killer is Hans Beckert (Peter Lorre), who lures children to their deaths.
Though the story is about a child killer, the theme of "M" centers more on society's obsession with the murders. From kids playing games, to adults going about their everyday lives, everyone is on guard, suspicious, combative, and very vindictive. For selfish reasons, even the petty criminal underworld wants the child killer caught. But the police are ineffective, and a self-righteous mob mentality takes over. The whole film is really a study in the terrifying psychology of "group think".
As it turns out, the killer is less monstrous than pathetic and compulsive. Beckert can't help himself. He's a short, squatty, bug-eyed little man who can't control the devil within. Lorre was perfect for the role, and his acting is flawless and mesmerizing. Yes, the killings, all off-screen, are awful. But the militant lynch-mob mentality is just as scary. Beckert is the only individualist in the film.
Part silent film and part talkie, the B&W visual style of "M" is wonderfully expressionistic and baroque. There are lots of overhead camera shots. Background sounds, like honking horns and whistles, are precursors of major plot points.
What strikes me most about this film is its tone. I can think of very few films that are so harsh, so bitter, so malicious, so punitive. "M" contains not an ounce of humanism. Lang's ex-wife, Thea von Harbou, a Nazi sympathizer, wrote the film, just before the rise of Hitler. The film's angry, militant tone thus foreshadows Hitler's Third Reich, from 1933 until 1945. (Lang himself fled Germany, and ended up in the U.S.)
Your own personal preferences in films will determine whether you consider "M" to be a masterpiece, or overrated, or perhaps somewhere in between. But individual preferences aside, there can be no doubt that "M" is an important film, historically. Every person who wishes to be versed in classic films needs to see "M" at least once.