There is a strange continuity in German movies: about every 20 years someone makes a film about a serial-killer. Apart from "Es geschah am hellichten Tag" (recently remade by Sean Penn) I'm thinking of the following works:
* M - Eine Stadt sucht einen Mörder (1931)
* Nachts, wenn der Teufel kam (1957)
* Die Zärtlichkeit der Wölfe (1973)
* Der Totmacher (1995)
While three of these films are more or less loosely based on the case of Fritz Haarmann who killed more than 24 young men in the 20s, "Nachts, wenn der Teufel kam" is about Bruno Luebke who murdered several people in Hamburg during WWII (also a true case). In contrast to the picture that many American movies (e.g. "Hannibal") paint of a serial-killer as an evil being who kills for pleasure, these German movies show men who are helpless victims of their urge to kill, to which they succumb not when they want to, but when they 'have' to. Mario Adorf plays Bruno as such a man and his performance is of the same rank as Peter Lorre's in "M" or Götz George's in "Totmacher" IMO.
Even better is Hannes Messemer as an SS-Officer, who, for 'political' reasons, wants another man executed against better judgement. The main forte of the film however, is the depiction of everyday-life in the last years of the third Reich. In the scene where the ugly harvest helpers get their reward from a sweating hanger-on Robert Siodmak perfectly captured the moral corruption (thinly veiled by empty propaganda phrases) within Nazi-Germany. In view of mass-murder of an entirely different caliber (i.e. genocide), the question if the right man is sentenced for a killing series becomes secondary in the end.