Der Verlorene aka The Lost Man (1951)

Directed by Peter Lorre. 

Based on a novel by Peter Lorre screenplay was by Axel Eggebrecht, Helmut Käutner, Peter Lorre and Benno Vigny. Cinematography was by Václav Vích, Music by Willy Schmidt-Gentner.

The film stars Peter Lorre as Dr. Karl Rothe, aka Dr. Karl Neumeister, Karl John as Hösch, aka Nowak, Helmuth Rudolph as Colonel Winkler, Johanna Hofer as Frau Hermann, Renate Mannhardt as Inge Hermann, Eva Ingeborg Scholz asUrsula Weber, Lotte Rausch as Woman on Train, Gisela Trowe as Prostitute, Hansi Wendler as the Secretary.

This story is told in a series of flashbacks. Dr Rothe is a kindly German doctor caring for refugees and displaced persons in a relocation camp after the war. When Hösch his former assistant is brought to the camp he triggers the flashbacks.  

Karl John as Hösch

Peter Lorre as Dr. Karl Rothe, aka Dr. Karl Neumeister

In reality he is Dr. Karl Neumeister who was a scientist doing bacteriological research to help the Nazi war effort during WWII. 

All is good until the SS informs him that his fiancée is secretly selling his secret discoveries and formulas to the Allies for food money while also having an affair with Hösch. Hösch is an SS informer who reveals her activities to the authorities. Neumeister goes off his rocker and murders her by strangulation. 

The crime is covered up by the SS. However  things go Noirsville when Neumeister not quite right in the head any longer now gets an un-controllable impulse to strangle any woman who reminds him in any small way of his dead fiancée.

Noirsville

Lorre has some great closeups, his puss twitching grotesquely in this depicting his conflicting emotions and tortured mental illness. Václav Vích's  cinematography is both nightmarish and dream like. A reviewer below sums the film up pretty well.

Classic film noir by an unexpected master

"After years of dreary labor in Hollywood as a professional "evil foreigner," Lorre went home to Germany to write, direct and star in this dark, dreamlike narrative in which he plays the ultimate Peter Lorre character: a Nazi mad doctor sex murderer. The film is an ironic commentary by Lorre, the reluctant impersonator of psychopaths, on the nature of true psychopathology as embodied in the amoral Nazi regime. It's also an ingenious melding of the sort of B-film noir that Lorre had specialized in for years as an actor (Maltese Falcon, Stranger on the Third Floor, Quicksand) and the impressionistic Nouvelle Roman/Nouvelle Vague influenced art film just picking up steam on the continent (shades of Orpheus, Wild Strawberries, and Last Year at Marienbad can be seen in its shadowy enfolding of past/present and dream/reality.) Though somewhat uncertain in balancing himself between his roles as principal actor and director (the motivations of some of the other characters are somewhat murky, for instance, and it's rather a shock to see Peter Lorre so continually being the object of women's lustful attentions) this was clearly a man with the makings of an ingenious and original filmmaker. It's a shame this film isn't better known, and that Lorre never got the chance to make another." (IMDb Anne_Sharp 27 December 1999)

8/10 Screecaps from You tube streamer.