Sforno on Genesis 4:15/ספורנו על בראשית ד׳:ט״ו

1כל הורג קין שבעתים יוקם, I, G’d, announce that anyone who dares to kill Kayin, [i.e. who takes the law into his own hands, Ed.] would be punished seven more times than what the crime he committed appears to warrant. Anyone who is prepared to kill someone is called הורג, as for instance, in Hoseah 9,13 ואפרים להוציא אל הורג בניו, “Ephrayim too must bring out his children to his slayers.” The word שבעתים means “twice seven times,” as in Proverbs 6,31 ונמצא ישלם שבעתים, “when he is found out he must pay twice sevenfold.” G’d decreed therefore that when someone killed half the human race, or at least a third of the human race, the appropriate vengeance would involve a twice sevenfold penalty. This would manifest itself in such a slayer being a restless wanderer on earth for seven generations. Such a life is considered worse than death. When David prays in Psalms 59,12אל תהרגם פן ישכחו עמי הניעמו בחילך, “do not kill them lest my people will forget them; make them wanderers seeing it is in Your power,” he meant that they be killed only at the end of seven generations. David wanted the punishment of his enemies to last. If they would all die and disappear at once their absence would soon be forgotten. Kayin got his deserts at the end of seven generations as we know from traditional sources in Tanchuma 10.