Karl Marlantes, was the author of Matterhorn: A Novel of the Vietnam War published in 2010 that was called by Sebastian Junger “one of the most profound and devastating novels ever to come out of Vietnam.” The novel is based on his combat experience in the war. He was a frequent commentator in the television series. He was a significant contributor to the television series The War in Vietnam shown recently on PBS and produced by Ken Burns and Lynn Novice.
After the war he experienced, like to many other soldiers, post traumatic stress disorder. He said, “One of the things I learned in the war is that we are not the top species on the planet because we are nice. People talk a lot about how the military turns kids into killing machines and I will always argue that it is just finishing.
Gwynne Dyer, who was not a commentator in the series, but he had some things to say that I think are relevant and interesting. He pointed out that what sets soldiers apart from other groups of was that they have to be willing to kill. Yet as Dyer said, comparing soldiers to gangs:
But it is not a willingness that comes easily to most men—even young men who have been provided with uniform, guns, and official approval to kill those whom their government has designated as enemies. They will, it is true, fall very readily into the stereotypes of the tribal warrior group. Indeed most of them have had at least some glancing acquaintance in their early teens with gangs (more or less violent, depending on, among other things, the neighborhood), the modern relic of that ancient institution.
And in many ways what basic training produces is the uniformed equivalent of a modern street gang: a bunch of tough, confident kids full of bloodthirsty talk But gangs don’t actually kill each other in large numbers. If they behaved the way armies do, you’d need trucks to clean the bodies off the streets every morning. They’re held back by the civilian belief—the normal human belief—that killing another person is an awesome act with huge consequences. 
So people as a rule have to be taught to kill. They have to be taught to ignore their “normal” instincts not to kill people. Armies expect that when the times come, their soldiers will not hesitate to kill the designated enemy. That is not as simple as it might sound.
Armies actually contain fairly normal ordinary men and women. Such people find it difficult to kill in most circumstances. They have to be persuaded to kill. Armies always assumed their soldiers would kill when they had to.
The Americans decided to check in the Second World War. Were their soldiers actually killing as required? US Army Colonel S. L.A. Marshall actually looked into it and what he found surprised him and many others. He found that in 1943-1945 on average only 15% of trained combat riflemen actually fired their weapons during battle! The rest of the soldiers by and large did not flee or desert. They just did not fire their guns even when their own position was under attack and their own lives and that of their comrades were in danger! This was true whether the action was spread over a day, or a few days. In very aggressive companies the percentage rarely exceeded 25%. Another interesting fact, according to Dyer’s reading of the Marshall’s research, each man (they were mainly men) thought he was the only one not firing. Soldiers did fire if they were with other soldiers because they did not want to be seen holding back, but when alone most did not fire.
There is no similar problem with artillery soldiers or bomber crews. It is thought that this is because they are far enough away that they cannot see their victim. The victim is not real to them. According to Dyer, “they can pretend they are not killing human beings.”
After that, the Americans stepped up their training to get more to kill. As a result it was found in a similar test in the Korean War that 50% of such soldiers fired their weapons. I don’t have the figures for the War in Vietnam. Ye tit is clear indoctrinating soldiers to kill helps “improve” the odds that they will kill.
In the end in the Vietnam War there was plenty of killing. Before the war was over more than 58,000 Americans would be dead, at least 250,000 South Vietnamese troops died, in the conflict as well. So did over a million North Vietnamese soldiers and Vietcong guerillas.” Added to that, 2,000,000 Vietnamese civilians are thought to have died as well as tens of thousands in neighboring states such as Laos and Cambodia. Vietnam actually lost 10% of its population in the war. That is a lot of killing.
 Gwynne Dyer, War (1985) p. 116
 Gwynne Dyer, War (1985) p. 118
 Geoffrey Ward, The Vietnam War (2017) produced by Ken Burns and Lynn Novick, PBS