Charles Ellicott Commentary Genesis 9:6

"Whoso sheddeth man`s blood, by man shall his blood be shed: For in the image of God made he man." — Genesis 9:6 (ASV)

By man ... — This penalty of life for life is not to be left to natural law, but man himself, in such a manner and under such safeguards as the civil law in each country directs, is to execute the Divine command. And thus protected from the violence of both humans and animals, and with all such terrible crimes forbidden that had polluted Adam’s beginning, Noah in peace and security is to begin anew mankind’s great work on earth.