The Unfaithful Wife (1969)

Charles Desvallées has good reasons to believe that his wife is cheating on him and hires a P.D. in order to prove himself right. Once he knows the lover is writer Victor Pégala, he drives to his apartment, calmly presents himself as the husband, starts a conversation and then kills him cold-bloodedly. The police trace the wife but when she discovers by accident a picture that could incriminate her husband she decides to remain silent.