Everyone except the abnormally saintly or submissive possesses the retaliatory instinct. It lurks like a small black gland at the base of the brain, in the mind's nonreasoning regions. When a person's elemental sense of justice is offended, the retributive instinct flares and hops in outrage; it gesticulates like Mussolini; it demands satisfaction. The urge is deep and primitive. Some cannibals on Pacific islands used to eat convicted murderers for dinner—a practice that appeased both their hunger for food and their thirst for justice.
Over many months, the American retributive gland has grown more and more inflamed. A few weeks ago, Robert...