For it is not the part of a righteous judge to define requital on the principle of like for like, but the originator of evil must pay his debt with addition, if he is to be made better by punishment and render other men wiser by his example.  Therefore, since it is ordained that Cain pay the penalty of his sin sevenfold, he who kills him, it is said, will discharge the sentence pronounced against him by the divine judgment.  This is the sense that suggests itself to us on our first reading the passage.