Slowly drive the person insane (let's assume it's a him), once insane enough to resort to murdering anyone and/or cannibalism, set him free to kill his relatives. If he succeeds, deprive him of food so much so that he resorts to self-cannibalism until he dies. If he does not succeed, his family members will be forever scarred with the images and memories of their beloved relative trying to eat them alive. The ultimate punishment in death is to make them suffer the greatest amount of pain, both physical and psychological, before death, or to have them be remembered as horrible and appalling to all, especially people who were close to him.