a film that is primarily a detective story that portrays the killer as a grotesque usually hidden in black and wielding a nasty weapon like a knife, axe, even a clawed hand

Giallo (pronounced in Italian: approximately JAH-loh) is an Italian 20th century genre of literature and film. It is closely related to the French fantastique genre, crime fiction, horror fiction and eroticism. The term is also used to mean an example of the genre, in which case it can take the Italian plural gialli.