Malin: It is the purpose or "nature" of war to kill, wound, cause a surrender or defeat, etc.
In the rules of war, the difference is, when a combatant surrenders, and lays down his weapons; then a crime is committed if the combatant is then killed after surrendering. When there are several killed in this manner, this constitutes a massacre. As in My Lai, VietNam, "The Crater", Saltville, Fort Pillow, Port Hudson, Olustee, Millikens Bend and others.
Dictionary version is: The act or instance of killing a number of human beings indiscriminately and cruelly.
You know this; so what is your point?