In countries where the usual punishment for such crimes is a lengthy term of imprisonment, the operating theory that justifies that cruel punishment is that it will prevent more crimes being committed — both by the imprisoned, who is kept safely away from people we care about, and by the victims who might otherwise take matters into their own hands and start an eternal murderous blood feud if their urge for revenge isn’t satisfied by the courts. Such arguments start to look increasingly indefensible in any case, no matter how heinous was the crime, as the decades go by without such events.