Is there a set of rules for war?

The rules of war, sometimes known technically as international humanitarian law, are a set of international norms that govern what can and cannot be done during an armed conflict. The primary goal of international humanitarian law (IHL) is to keep humanity alive in armed conflicts by saving lives and alleviating suffering.

Is it a war crime to kill civilians?

Willful killing, or intentionally killing civilians, as well as "willfully causing great pain or serious harm" when injuring victims, are both considered War and Crime. Individuals who commit, order or condone war crimes are individually accountable for their crimes under international humanitarian law.