Genesis 4:15

15. Therefore, whosoever slayeth Cain. They who think that it was Cain’s wish to perish immediately by one death, in order that he might not be agitated by continual dangers, and that the prolongation of his life was granted him only as a punishment, have no reason, that I can see, for thus speaking. But far more absurd is the manner in which many of the Jews mutilate this sentence. First, they imagine, in this clause, the use of the figure ἀποσιώπησις, according to which something not expressed is understood; then they begin a new sentence, ‘He shall be punished sevenfold,’ which they refer to Cain. Still, however, they do not agree together about the sense. Some trifle respecting Lamech, as we shall soon declare. Others expound the passage of the deluge, which happened in the seventh generation. But that is frivolous, since the latter was not a private punishment of one family only, but a common punishment of the human race. But this sentence ought to be read continuously, thus, ‘Whosoever killeth Cain, shall on this account, be punished sevenfold.’ And the causal particle לכן (lekon,) indicates that God would take care to prevent any one from easily breaking in upon him to destroy him; not because God would institute a privilege in favor of the murderer, or would hearken to his prayers but because he would consult for posterity, in order to the preservation of human life. The order of nature had been awfully violated; what might be expected to happen in future, when the wickedness and audacity of man should increase, unless the fury of others had been restrained by a violent hand? For we know what pestilent and deadly poison Satan presents to us in evil examples, if a remedy be not speedily applied. Therefore, the Lord declares, if any will imitate Cain, not only shall they have no excuse in his example, but shall be more grievously tormented; because they ought, in his person, to perceive how detestable is their wickedness in the sight of God. Wherefore, they are greatly deceived who suppose that the anger of God is mitigated when men can plead custom as an excuse for sinning; whereas it is from that cause the more inflamed.

And the Lord set a mark. I have lately said, that nothing was granted to Cain for the sake of favoring him; but for the sake of opposing, in future, cruelty and unjust violence. And therefore, Moses now says, that a mark was set upon Cain, which should strike terror into all; because they might see, as in a mirror, the tremendous judgment of God against bloody men. As Scripture does not describe what kind of mark it was, commentators have conjectured, that his body became tremulous. It may suffice for us, that there was some visible token which should repress in the spectators the desire and the audacity to inflict injury.