The Strange Vice of Mrs Wardh (1971)

What it's about

When socialite and heiress Julie Wardh begins receiving blackmail letters attributed to a mysterious serial killer, she suspects her cruel and sadistic former lover Jean is behind them. With her husband Neil frequently out of town, she falls into the arms of her friend's cousin George, and as the unknown assassin begins to make his move, she fears that one of the three men in her life may be the killer.