"it is justifiable to use weapons on unarmed civilians ONLY in a...

"it is justifiable to use weapons on unarmed civilians ONLY in a war zone, provoked or otherwis"

No, under the Geneva Convention and Hague Law it is illegal to use weapons on unarmed civilians.

There are grey areas and exceptions but the general idea is that deliberate targeting of civilians, or use of them as human shields is in contravention of the rules of war.

You can accidentally kill civilians if you did not know they were there, or if your life is under threat from an enemy positioned near civilians.

There are also laws about proportionate response, ie. you can't indiscriminately destroy buildings, target hospitals or schools. Blowing up a building to kill one sniper is considered disproportionate force.

That said, it often hard to bring the hammer down on anyone.