The reason can be summed up in either two ways.
The first is fear. Fear of the unknown, fear of the armed individual in front of you whom you have never met, and don't know, and cannot trust - because if you trust them at the wrong time, you are dead. And everything you worked for is gone and lost forever. That, right there, is a genuine fear of loss, a knowledge of danger and threat that makes the heart race and the mouth run dry. During such moments of fear, if fear is felt, an individual will, unless they learn to control that fear, respond violently. If that response meets with a positive effect ("the other individual is dead, I am still alive, therefore this was the correct decision" sort of scenario) and thus, the fear is neutralised, they will continue to respond to the positive stimulus, and become more and more relaxed about shooting the other individual.
Why is there fear? You speak of armies, of war, but this isn't war. It is survival. In war, there are sides, there are uniforms, there is a way to identify who is friendly and who is not. But this isn't war. This is survival. The closest you can come to a war in this description is that it is an insurgency uprising, because there are no uniforms, no way to identify friend from foe, no way to know if the person you are talking to is on your side, or a psychopath.
The second reason is the result of the times we live in, and mild (to severe) sociopathy. They do it because for them, it is fun. It is an outlet of spite, of viciousness, a 'harmless trolling' activity, which they enjoy. This is a harmless environment for them to do so, as no actual physical trauma is inflicted, and whatever mental trauma, if any, that is inflicted is only temporary. These are individuals who may not necessarily be socially awkward, or misanthropic, but the reason they do it is because they enjoy making others suffer. If they do it to survive, they are not part of this group. But if they do it to people who are absolutely no threat and they gain nothing from it, that is just so they can experience murder vicariously.
That is not to say they are murderers in the making. They are not. This is, above all else, a game. They do not actually harm another human being. They do not actually kill someone. In this circumstance, it is a harmless activity that has no real emotional impact, and if confronted with actual, brutal violence, they will be (in most cases) sickened and appalled. It is, in effect, an outlet for sociopathic tendencies, which, in truth, most people have. It is more a symptom of growing societal apathy and disassociation from consequence and make believe violence combined.
Day Z is a perfect for the above circumstance. It promotes societal apathy (better him than me), the violence is make believe and fake (no one -really- gets hurt) and it removes consequences (no one will punish, arrest, or ban you for shooting someone. It is in fact encouraged).
It is, in short, a game perfect for trolls, as well as normal people. Both draw equal enjoyment out of it. But in many cases, for entirely different reasons.
Oh, and of course, when you get good enough gear, and guns, and are practiced at the game enough - hunting players is the endgame of Day Z. Its the natural challenge progression.
EDIT - Since you said highbrow book learning, I thought I'd throw my own into the hat
