The Incredible 2-Headed Transplant(1971)

Dr. Roger Girard is a wealthy scientist who conducts experiments on head transplants. His caretaker has a son named Danny. Even though Danny is an adult, his mind is like a child. One day, a psychopathic killer attacks Girard's house, kills Danny's father, and is shot himself. Danny is saddened by his father's death, so Dr. Girard decides to transplant the head of this murderer to Danny's body. The situation is out of control and this creature is running away from the place...