There is advantage to peace. It's prudent to discourage killing, even if it's not provided by supernatural decree.
This is fundamental political theory stuff. All men are not created equal, their collective capabilities are pooled together in society and divided by society, according to the liberties that society grants. Without such redistribution, we'd be all subject to the single Goliath of a society. Instead, the Goliath relinquishes some of his might, and the David is given access to it.