Part of the Rainbow covenant God made with Noah, before anything was written down in a book...just written down years later when anyone thought to do so from the oral traditions amplified by the revelation of God’s law to Moses!
Genesis 9:6 “Whoever sheds human blood,
by humans shall their blood be shed;
for in the image of God
has God made mankind.
That’s the earliest command against murder, set down by direct edict by God himself, and he also sets down the moral “why” we don’t murder, which darwinists and those who try to derive social policy from evolutionary theory have no answer for...that is “for in the image of God has God made mankind”!
If there is no God, there are no overarching realities that would give the yea or nay to our destructive impulses. Or, as Paul states, “If there be no resurrection of the dead, let us eat, drink, and be merry for tomorrow we die!”