'That simple' is very naive and unrealistic. For one side to do no harm to others but allow the other side to harm them is anti-survival. This is a rough world where we do have to act to survive against predators.

You want 'natural law'? The natural law built in by genetics is to survive. It drives predators to attack. If dealing with that means killing predators attacking you, the winners survive and go on. The losers are Losers with a capital L. THAT is natural law, not some lovey dovey hippy peace shit.