Something doesn’t have to be illegal to be criminal. […] This word is being used with the definition of “A serious offense, especially one in violation of morality.”

we’re talking about war crimes not crimes in general. a war criminal is someone who commits war crimes. according to britannica a war crime is a “serious violation of the laws or customs of war as defined by international customary law and international treaties.” this has nothing to do with morality it is pure jurisprudence.