Another Man's Poison (1951)

Mystery writer Janet Frobisher lives alone in a dark English country house, when she's not philandering with her secretary's fiancée. At an extremely awkward moment, she has an unwelcome visitor: George Bates, who claims to be the partner in crime of Janet's estranged husband. George insinuates himself into Janet's home and life despite her efforts to get rid of him; the tangled relationships develop into a macabre, murderous cat-and-mouse game.