The vast majority of those that sign up for service in any of the American military service branches are in the age bracket of 18-24, of which most of those recruits, are male, in which, history has repeatedly demonstrated that males are far more aggressive than females in regards to violence and taking violent action. That said the respective military branches aren't looking for recruits that have previously been convicted of murder or manslaughter, for though that demonstrates their killing ability, those kills have been accomplished without appropriate state sanction. Still, no doubt, there probably are a very minute percentage of recruits that have actually killed another human being, but have done that in legitimate self-defense. So then, it is fair to state, that those that join the military have in virtually every instance, never killed another human being, though there probably are a significant percentage of military recruits that have killed animals, such as in hunting activities or similar.
This does mean that military recruits are completely unfamiliar with the killing of human beings, before joining the military. Warfare, however, in and of itself, when looked at in its essence, is the government-sanctioned killing of other human beings, of which, those other human beings, are often demonized, re-branded as sub-human, defined as enemies, and seen as a clear and present danger to the state so that they must be killed. So too, the enemy is seldom regarded as actually being human and as being created equally by God, but rather are subject to semantic word changes, which designates them as targets, which should be engaged; when in reality it is really about the deliberate killing of other human beings, designated as the enemy.
Not too surprisingly, the average human being, does not have a natural affinity towards killing other human beings, especially when doing so, is not in the defense of their family, or their property, or their self, or has come about through "crimes of passion" or similar. Those that run the military establishment are well aware of this, and further are well aware that the physical distance from an intended human target is highly relevant, in addition to the known fact that those under orders to an established authority are typically obedient to that authority, that therefore enables soldiers to circumvent whatever personal moral qualms that a given soldier may have when it comes to the killing.
So too, the military is very good about telling bombardiers as well as all those soldiers that are operating mortars, cannons, and other long range weaponry that they are specifically shooting at other ships, or bombing buildings, or destroying infrastructure, without bothering to discuss that within those ships, buildings, and infrastructures are actual human beings. Additionally, the greater the distance between a soldier and the human being that has been targeted to be killed, the less visceral and real the feel is, especially if in order to accomplish the killing, this merely requires the pressing of a trigger or a button, in which, therefore the weapon provides the actual killing power, as opposed to the soldier actually having to physical penetrate another human being's body with a weapon such as a knife or saber.
The military does a lot of things, some beneficial and some not, but none should be surprised, that when you train a soldier to kill another human being, which is an unnatural act, that the returning solder ends up, so often, not being right in their head, for they know, at the end of day, the difference between wrong and right.