Genesis 9:6 - Joseph Benson’s Commentary on the Old and New Testaments

Whoso sheddeth man's blood Whether upon a sudden provocation, or premeditated, (for rash anger is heart-murder, as well as malice prepense, Matthew 5:21-22,) by man shall his blood be shed That is, by the magistrate, or whoever is appointed to be the avenger of blood. Before the flood, as it should seem by the story of Cain, God took the punishment of murder into his own hands; but now he committed this judgment to men, to masters of families at first, and afterward to the heads of countries. For in the image of God made he man Man is a creature dear to his Creator, and, therefore, ought to be so to us: God put honour upon him, let us not then put contempt upon him. Such remains of God's image are still even upon fallen man, that he who unjustly kills a man, defaceth the image of God, and doth dishonour to him. And what then shall we say of those who commit wilful and deliberate murder in duels? And what shall we say of the magistracy in any country that does not suppress this diabolical practice?