I commend your advocacy of war in all sincerity. You pose a valid point in that we cannot know peace without war. That is simply Nature for us. Jachin & Boaz, Yin & Yang, Life & Death, Male & Female, Light & Dark, Day & Night. Good & Evil, Active & Passive are all concepts in nature seen as pairs of opposites the same as War & Peace, and we cannot know one without its opposite. While appreciative of this I have difficulty accepting war serves what many deem a "good" purpose.
Though I suppose the argument of thinning the herd might be a "good" purpose. I know here in WV we have ad hoc deer hunting seasons which are perfectly legal, they get done to thin the herd of deer. If deer over populate an area, deer die off because the area can only support so much deer population. Perhaps the same is true of human beings.
This noted it would seem a more rational means, more humane means could exist to achieve the same ends. Do I know what that could be? No. I make no claim of having all answers or knowledge. In fact there is much I simply rather not know. I find that in knowing comes some anguish at times, for example once you learn how magicians do tricks, you lose capacity to know magic, wonder. So there is a lot I rather not know.
My point being, I'm kind of in agreement with
@The Cromwell on self defense being the only just motivation to kill. I will also though include defense of those you love as a motivation, justification. Do I hold to capital punishment? Only in extreme cases ruled so by society's common good and sense of justice (note I do not say vindication, vengeance here, I explicitly say justice).