Whosoever slayeth Cain, &c. God having said, in Cain's case, Vengeance is mine, I will repay, it had been a daring usurpation for any man to take the sword out of God's hand. And the Lord set a mark upon Cain To distinguish him from the rest of mankind. What the mark was, God has not told us: therefore the conjectures of men are vain.
Related Commentaries of Genesis 4:15
Genesis 4:15
15 And the LORD said unto him, Therefore whosoever slayeth Cain, vengeance shall be taken on him sevenfold. And the LORD set a mark upon Cain, lest any finding him should kill him.