Question: "Why did God mandate capital punishment for murder (Genesis 9:6)?"
Answer: After Noah, his family, and the animals exited the ark, God gave a new command: put to death anyone who murders another person. Genesis 9:6 says, “Whoever sheds the blood of man, by man shall his blood be shed, for God made man in his own image.” The severest of penalties is to follow murder, and God Himself gives the reason for it.
God specified that murder was to be punished by death because of the nature of man. Man is created in God’s own image (Genesis 1:27). As murder destroys an image-bearer, it is a direct affront to God Himself. Humans are unique among God’s creations—none of the animals are created in God’s likeness—and murder is a unique crime.
Another, secondary reason for the mandate is quite practical. The immediate context includes another command given to Noah and his three sons: “Be fruitful and multiply and fill the earth” (Genesis 9:1). Murder, of course, would work against humanity’s being fruitful and multiplying. The death penalty for murder thus served as a deterrent to anyone who sought to thwart God’s plan to replenish the earth. This was especially important when Noah’s family first departed from the ark, at which point only eight people were alive.
Before the Flood, Cain had murdered Abel, and, although Cain was judged by God, he was not put to death (Genesis 4). Lamech, a descendant of Cain, also murdered someone (Genesis 4:23-24). By the time of God’s judgment in Genesis 6, it appears that crime was rampant, including the crime of murder. After the Flood, a new standard was raised as part of the recreated earth: God would no longer tolerate murder. Later, murder was condemned in the Ten Commandments (Exodus 20). The punishment for premeditated murder was death (Numbers 35:30-34).
In the New Testament, Jesus provided a wider application of the Old Testament command against murder. He taught, “You have heard that it was said to those of old, ‘You shall not murder; and whoever murders will be liable to judgment.’ But I say to you that everyone who is angry with his brother will be liable to judgment; whoever insults his brother will be liable to the council; and whoever says, ‘You fool!’ will be liable to the hell of fire” (Matthew 5:21-22). Murder is wrong, and the attitude behind the action is just as wrong. God sees the heart and its intentions (1 Samuel 16:7).
Murder is consistently listed as a sin throughout the New Testament (e.g., Revelation 22:15). Man still bears the image of God, and God’s view of murder has remained the same.