Another Man's Poison (1951)

"She Had Everything You Could Give A Woman To Torment A Man!"

Mystery novelist Janet Frobisher, lives in an isolated house, having been separated for years from her criminal husband. She has fallen in love with her secretary's fiancé and when her estranged husband unexpectedly returns, Janet poisons him, but just as she's about to dispose of the body, one of her husband's criminal cohorts also shows up.