There are some parts of M, both in form and in content, that can only be explained by the time and place of its production. A child murdered would not fit easily as a subject not even in contemporary films (cannibals seem to be okay nowadays, but child murderers are not so prominent). Early 30s was a time of disillusionment for Germany which lived the last days of the Weimar republic just before the ascend of the nazis to power a couple of years later. The main theme of the film is not the specific target of the murderer (Lang only needed something atrocious) but the general feeling of insecurity, criminality, disorder, and state failure. In this, Lang and his wife Harbow (at the time a nazi sympathizer, later a member of the party) follow the same general rhetoric which was so much exploited by the law-and-order obsessed nazis. So the state in the film has its hands tied and cannot do its job in time (ie before the killer strikes again) because of red tape and those in high places who are incompetent, a major trope of police procedurals ever since. The vacuum caused by this incompetency must be filled: that's where the syndicate of beggars comes in -- and it should be noted that it does a great job as an alternative state apparatus, a strange and unexpected fiction (beggars conducting police work and orderly trials?) which is nevertheless believable, since that's exactly what happened 2 years later. The grim and dark subject, the oblique, unexpected, unnatural corners of the camera, the strong contrast, all of them inherited by classical american noir, are all children of an oblique, unnatural, dysfunctional society, Weimar Germany. And it would be interesting to try to understand why such dark themes, both in content (murder, fate, the inability of the main character to transcend his/her vicious nature) and in form (dark, contrasted, low key etc) would resonate with audiences living in post war societies. Or, if one considers the subject of super-villains in superhero films as the inheritors of the almost all-powerful beggar syndicate, with audiences today.