Silent Night, Bloody Night (1974 Public Domain)

In 1950,Wilfred Butler runs out of his house and has been set on fire. He collapses in the snow and is believed to be dead.
Years later, lawyer John Carter and his assistant Ingrid arrive in a small Massachusetts town. John meets with the town's nobles, Mayor Adams, Tess Howard, Sheriff Bill Mason and Charlie Towman. John is revealed to be the lawyer of Jeffrey Butler, Wilfred's grandson, and is trying to sell the Butler mansion for $50,000 by noon the next day. After a call to his wife, John is revealed to be having an affair with Ingrid. The two stay the night at the Butler mansion, unaware that they are being watched. After dinner, the two go upstairs to a bedroom to have sex. The unknown person walks in on them and kills them both. The killer calls the police and reveals himself to be Marianne.