Genesis 9:5

Gen. 9:5, etc. "And surely your blood of your lives will I require it - whoso sheddeth man's blood, by man shall his blood be shed." We have an account of murders before the flood, but nothing that looks as though murder was wont then to be revenged with death by men, in an established course of public justice. Lamech, when he had been guilty of murder, seems not to have been executed for it by men. And by the story of Cain, it should seem that God took the punishment of murder then into his own hands. In all probability, a little before the flood, when we read that the earth was filled with violence, the earth was filled with murders, and that those giants who then became such mighty men, and men of renown, were guilty of many murders, and that it was in the earth as it was in corrupt times in Israel, and the land was filled with oppression and violence, in other respects their hands were full of blood, Isaiah 1:15; Jeremiah 2:34, "And the land was full of blood," Ezekiel 9:9. By swearing, and lying, and killing, and stealing, and committing adultery, they broke out, and blood toucheth blood: the like in many other places. And there being no human laws for putting murderers to death, therefore God did in a remarkable manner take that work into his own hands in the destruction of those murderers by the waters of the deluge; but now establishes it as a rule henceforward to be observed, that murder shall be revenged in a course of public justice.

Another reason why God now does expressly establish and particularly insist on this rule is, that God had now first given them leave to shed the blood of beasts for food, which had not been granted till now, which liberty they would have been in danger of abusing, to make shedding of blood appear a less terrible thing to them, and so taking encouragement the more lightly to shed men's blood, had not God set up this force.

Gen. 9:12