From Lt. Col. Dave Grossman’s book, On Killing.
“There is such a thing as a "natural soldier”: the kind who derives his greatest satisfaction from male companionship, from excitement, and from the conquering of physical obstacles. He doesn’t want to kill people as such, but he will have no objections if it occurs within a moral framework that gives him justification - like war- and if it is the price of gaining admission to the kind of environment he craves. Whether such men are born or made, I do not know, but most of them end up in armies (and many move on again to become mercenaries, because regular army life in peacetime is too routine and boring).
But Armies are not full of such men. They are so rare that they form only a modest fraction even of small professional armies, mostly congregating in the commando-type special forces. In large conscript armies they virtually disappear beneath the weight of numbers of more ordinary men. And it is these ordinary men, who do not like combat at all, that armies must persuade to kill. Until only a generation ago, they did not even realize how bad a job they were doing.“
"One veteran I interviewed told me that he thought of most of the world as sheep: gentle, decent, kindly creatures who are essentially incapable of true aggression. In this veteran’s mind there is another human subspecies (of which he is a member) that is a kind of dog: faithful, vigilant creatures who are very much capable of aggression when circumstances require. But, according to his model, there are wolves (sociopaths) and packs of wild dogs (gangs and aggressive armies) abroad in the land, and the sheepdogs (the soldiers and policemen of the world) are environmentally and biologically pre-disposed to be the ones who confront these predators.”
These sheepdogs are estimated to be about 2% of the male population. Just one of many nuggets of interest that Lt. Col. Grossman filled his book with. If you’re looking for a book on a topic that will broaden your horizons in strange ways I’d recommend On Killing: The Psychological Cost of Learning to Kill in War and Society. The subject of Killology is fascinating, after all it’s like virgins studying sex.