Cast a Dark Shadow (1955)

Clever fortune-hunter Edward Bare (Sir Dirk Bogarde), with a penchant for murder, does in his elderly, supposedly rich, wife, and manages to get away with it. After an investigation results in a decision of "accidental death," Bare discovers that his late wife's "fortune" is not what he thought it was. Driven to find another unsuspecting spouse, he discovers that his new bride, a widow, is no fool. After she tells him that she intends to keep her accounts separate from his, he is driven to contemplate murder once again.