The Incredible 2Headed Transplant (1971)

Dr Roger Girard is a rich scientist conducting experiments on head transplantation His caretaker has a son Danny who although fully grown has the mind of child One day an escaped psychokiller invades Girards home killing Dannys father before being gunned down himself With the maniac dying and Danny deeply unsettled by his fathers death Dr Girard decides to take the final step and transplant the killers head onto Dannys body