A weird one.Nothing like this had been done before,and it has never been imitated since.Only Richard Fleicher's movies ("Follow me quietly" ,"the Boston strangler" and "10 Rillington Place") have something (very vaguely) similar.
It is a film actor Jacques Perrin undoubtedly made as an act of self-renewal.At the time ,he was probably sick and tired of the nice characters he played: the nice journalist in "Z" ,the nice sailor in "Les Demoiselles DE Rochefort" and the very same year as "L'Etrangleur" ,Prince Charming in "Peau D'Ane" (aka "Donkey Skin")! Playing a fairy tale prince and a strangler the one after the other was quite a feat for an actor!
Perrin was an amazing choice anyway:his boyish look was not generally scary.Because if you are looking for a horror movie,you have to move on. The strangler has developed "happiness sickness" and he kills only women who "look " unhappy,to help them.Why not men?,he's asked .He answers that he has never met a man with a miserable face in the dark streets of the town .A cop (who has just resigned ?) gets in touch with him and discovers he is followed by "the jackal" a man who steals money and jewels from the dead bodies.
It's a movie easy to admire but difficult to like.As for me,I like Fleischer's works best and in spite of its great qualities,it leaves me unconcerned .