This is a good philosophical question, but the answer the governments of major powers came up with is that to limit the atrocity there must be rules even in war

This is a good philosophical question, but the answer the governments of major powers came up with is that to limit the atrocity there must be rules even in war. So people came up with mutually agreed conventions. It was agreed that violating these would be worse then killing "by the book". Germany was a party to the rule prohibiting chemical warfare: https://verdragenbank.overheid.nl/en/Verdrag/Details/002422 https://verdragenbank.overheid.nl/en/Verdrag/Details/002422