Killer!

This Man Must Die, American title The Beast Must Die, is a 1969 French and Italian psychological thriller film directed by Claude Chabrol. The story is based on the 1938 novel The Beast Must Die by Cecil Day-Lewis, writing as Nicholas Blake. A widower, obsessed with revenge after his only son is killed in a hit-and-run incident, tracks down and seduces the driver's mistress who was in the car at the time but his efforts to kill the driver misfire.