Genesis 9:6

Whoso sheddeth man's blood, by man shall his blood be shed; for in the image of God made He man. While the blood and the life of animals is in the power of man, he is strictly forbidden to shed that of his fellow-man. The blood of every person with reference to his soul (since the life is in the blood) the Lord will require at the hands of man and of every beast. Thus the life of man is here safeguarded against beasts as well as against fellow-men. The killing of every human being will be punished by the Lord, but not directly or immediately, as He had promised in the case of Cain. He that sheds man's blood, by man shall his blood be shed. The punishment of murder is laid into the hands of the government, which shall punish the murderer by requiring his life in exchange for that which he took. This is, as Luther remarks, the first command regarding the authority of the government in the wielding of the sword. In these words the temporal government is authorized, and the authority from God to use the sword is conferred. For in the image of God made He man: murder is a violation of the image of God in man, which the Lord intends to restore in all those that are renewed in faith, and which He wants all men to put on. In a wider sense, therefore, man bears even now the image of God, since he is a rational creature and has an immortal soul.