John Wesley on Genesis 9:6

Whoso sheddeth man's blood - Whether upon a sudden provocation, or premeditated, (for rash anger is heart - murder as well as malice prepense, Mt 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 afterwards 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.