Yep, [@Keyguyperson] has basically gone into more detail on exactly what I was implying with that last comment of mine, and cool to see a mention about how in the those wars the opposing sides often ended up more or less interacting with one another. I've even seen information about how during cease fires both sides would actually eat together, though I wasn't given citation on that particular detail.

Dehumanizing the enemy to the point where the soldiers no longer actually acknowledge that they are shooting at people just like them, or simply conditioning them with training to pull the trigger instinctively without even questioning the situation, is precisely how military works in the modern era. As science progresses, so does the understanding of how to brainwash people, which is why the way the military can control people like this developed. A lot of propaganda and media is also centred around facilitating this need for people to view the other side as something that isn't human, subsequently removing the effects killing them will have on the psyche.

Similarly, governments will make enemies out of nothing in order to start wars over materialistic agendas, not just ones rooted in prejudice or hatred, creating all sorts of justifications for why the enemy is the enemy and why you're a traitor if you don't fight the enemy. These people will never have to watch the people who die as a result of those conflicts they've instigated; the ordinary men and women and children will. This is another example of how becoming detached to things is the beginning of the end for someone having any semblance of being a real human.

Someone also doesn't need to see the thousands of people whose lives were taken by a nuclear weapon when compared to, say, seeing the person you shot with a gun, and guns are also largely varied with how much you need to actually see the person when they are killed. For example, a sniper can usually see their target in very vivid detail through a scope, so this type of person is more likely to be effected by having to shoot these people, though, like we've already said, the military does actually more or less drill out this tendency to think about what you're doing or feel anything about it, so the soldier is mostly operating on instinct at that point, or like a machine.

In earlier times it was a lot more difficult to condition soldiers to act this way, regardless of what culture or country they were part of, because weapons were significantly less impersonal and you were always forced to accept that this was someone just like you. It takes a whole lot more conviction to make yourself cut someone's whose looking you in the eyes down with a sword then it does to shoot someone whose basically a dot far away in the distance. This is why people usually had to believe they were fighting for a very good cause to be willing to kill like this, whereas other life forms will just kill out of instinct. Humans generally need a justification to ignore their conscience, but brainwashing can force your conscience to be locked away.

On the flip side of this though, and more focusing on what I believe to be the primary purpose of the original poster's question (how to write someone who handles taking a life for the first time badly), there are also countless stories of soldiers who have become shell shocked because of all the lives they had to take or all the death they had to witness. While nowhere near the same scale as the original poster's scenario of a single person taking the life of a single other person, it still shows the immense scarring that a person with a more tangible conscience, and general sense of morality and empathy, will go through from killing another.