Walk a Tightrope (1963)

Ellen (Patricia Owens) is happily married and very much in love with Jason (Terrence Cooper), but she is being blackmailed by her former husband as their annulment had never gone through. She sets up a complex plan with a hired gunman, Lutcher (Dan Duryea), to kill the ex-husband when he comes for some more money, but the current husband comes in unexpected and Lutcher, not knowing one from the other, kills him. Oops!