Silent Night, Bloody Night

Silent Night, Bloody Night Also released as Night of the Dark Full Moon and Death House is a 1972 American horror film directed by Theodore Gershuny and coproduced by Lloyd Kaufman. The film stars Patrick ONeal and cult actress Mary Woronov in leading roles, with John Carradine in a supporting performance. The plot follows a series of murders that occur in a small town on Christmas Eve after a man inherits a family estate which was once an insane asylum.

On Christmas Eve 1950, Wilfred Butler runs out of his rural house and appears to have been set on fire. He collapses in the snow and is believed to be dead.Twenty years later, lawyer John Carter and his assistant Ingrid arrive in a small Massachusetts town on Christmas Eve. John meets with the towns nobles, Mayor Adams, Tess Howard, Sheriff Bill Mason and Charlie Towman. John is revealed to be the lawyer of Jeffrey Butler, Wilfreds 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, his assistant. 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 mysterious person walks in on them and kills them both with a felling axe. The killer calls the police and reveals himself to be Marianne. ........

Source: Wikipedia