Hospital Massacre (1982) on Netflix

In 1961, young Susan (Elizabeth Hoy) and David are playing with a train with paper Valentine's Day hearts all around the room. A boy watching from outside leaves a red envelope at the front door and knocks. Susan answers, and she shows David and Valentine's card from Harold. The two laugh and crumple it up. Susan goes to the kitchen to cut some cake and when she walks back into the living room, David is dead and hanging on a coat rack. Susan turns and sees a smiling Harold standing by the open window. She screams and the young killer runs away.