This is one of many questions man has asked, repeatedly, over the ages. Philosophers, religions, science; all have tried to find the answer to that question. It does seem to be a fundamental part of nature, where three or more are gathered there will be conflict. I find that I like an old poem (I find claims of both Chinese and Japanese origin) that states it is as much our nature to fight as it is for fish to swim or birds to fly. Of course, that poem ends with what everyone agrees is the more important question: what do you fight for?
There are some in this world that are total pacifists; they would not lift hand against any living thing, even to save their own life. Violence done returns upon the actor many times over, they say. When we look at the effects of PTSD and its frequency in those who live "by the sword" in the old parlance, we see some justification for this belief.
And yet, we see that there are times when violence is necessary; The knight with sword and shield standing ready to protect the "Fair Lady"; the Samurai who stood to protect his village from bandits; the modern soldier fighting tyrants to free the oppressed, or battle the terrorists that seek slaughter under the guise of religion. In this next step, we find those that fight because they feel they must; that they are forced into battle to defend or protect. These warriors are slow to violence, but can justify truly brutal tactics and
actions if they feel the need. As their biggest drive is safety of their chosen protectorate, they can (and often will) find a way to justify whatever means are necessary in thier fight.
The next step down are a group of warriors very similar to those just described: those that defend an ideal; the "professional" soldier/warrior. While the idea may be (and usually is) quite noble, mankind is simply not psychologically capable of forming the same dedication, devotion, and loyalty to an abstract idea as we are to concrete concepts like "home", or "family", or even "country". With this separation of emotion, there is usually less severity of violence; unless the opposing force makes it personal, this is "just another day at the office." Conflict and danger are minimized whenever possible.
Some men fight simply like to fight. All they want is to see chaos and violence. These dogs are happy to kill, maim and destroy any- and everything they see. Ironically, the most vocal detractors of the armed forces of any nation seem unable to comprehend that we withing the military hate these monsters just as much, and try very hard to root them out of the service when found.
