Diabolique (1955)

Christina Delassalle suffers greatly at the hands of her brutish husband Michel. She inherited the boys' boarding school they run but it's clearly Michel who is in charge. She and Nicole Horner, one of the teachers and Michel's former lover, decide to kill him.

Christina, who has a serious condition, is terrified when, by chance, she meets a retired police inspector who decides to look into the case.