The Killer Is One of Thirteen (1973)

A recent widow has invited a group of family friends to her large and secluded country home. However, what the guests don't know is that the reason they've been assembled is that their host suspects one of them might be her husband's killer and she's intent on uncovering the identity of his murderer. As the guests begin to suspect each other, revealing long-kept and sinister secrets in the process, an unknown, black-gloved killer begins bumping them off in a variety of nasty ways.