I think that CS Lewis said
“I have often thought to myself how it would have been if, when I served in the first World War, I and some young German had killed each other simultaneously and found ourselves together a moment after death. I cannot imagine that either of us would have felt any resentment or even any embarrassment. I think we might have laughed over it (p. 119).” Mere Christianity by C.S. Lewis
Lewis thought war horrible, but not the worst thing that could happen to us. To kill or to be killed in war is not murder. Rather, war is a species of punishment that may require our death or the death of the enemy, but we cannot hate or enjoy hating those we kill.