'Blood Relatives' (1978) aka 'Laberinto Mortal'.

This oppressively creepy, sinisterly slashing, bone-fizzingly grim, Canadian-set murder mystery remains a darkly fascinating delight, so be sure to check it out if you dare to descend deep into the diabolically degenerated mind of one especially deviant killer!!! And nae bad it is too! (Personally, I'm rarely bored by murder maestro Claude!) Alongside the uber-talented character actor Donald Sutherland it also stars terror icon Donald Pleasence, sublimely gifted Gallic dream Stéphane Audran, and Deep Red's stilleto cool David Hemmings! Based on a popular crime novel by Ed McBain, Claude Chabrol's finely wrought, consistently engaging, impressively acted thriller is regarded by many cult film fans to be one of the very best McBain adaptations!