A Woman's Torment（1977）

Don and his wife Frances have been struggling in their marriage. Frances’s mentally ill sister, Karen, who lives with them, is taking a toll on their romance and as such, the Comptons have been considering having her committed to an asylum. Upon overhearing their plans, Karen runs away, taking refuge in an empty house on a remote beach. But slowly her grip on reality diminishes, giving way to increasingly violent delusions and an ever-growing bloodlust…