So I tried to gather the following into words, hopefully you'll understand. By default, keep in mind that what I will mention below speaks about most cases and that exceptions always exist.
During medieval times it were mostly mercenaries who were fighting for a king/themselves. It was their job, and that's all what they knew. I believe they were paid with salt as it was considered very precious during that time.
Humanity and morals weren't part of their job, and many of them would raid villages and rape people as-well as "victory" treat.
Of course there are some rare/individual cases, but if I am not mistaken that was the cases in most situations on the globe. We talk about the majority here after all.
Things changed especially during Napoleonic times (ignoring the 'heretic' crossbow era for now), when normal regular peasants could enlist and propaganda was used to spread in order encourage more people to enlist to the army - that was a whole new system. They were also fighting for a cause now!
The regular people are now part of the army, and not generation of special soldiers.
And with that, one thing led to another.
And if you go further back in history (and I forgot which historical figure said that), it was accepted that nomads would raid kingdoms and win, just to settle down and become a kingdoms themselves. This would make them content and "relaxed", allowing the next group to aggressively raid and take over them - just to repeat that cycle.
Eh, I guess what I try to say with all that is people who liked violence would practice it better than others and thus would have no problem preforming it and actually turn it into a "business". Today, we can see how brain beats brawn. You still practice a type of violence, but a different one - and this time for a different not selfish cause. People who didn't like violence but would fight for a certain cause would have these moral choices and would deal with it in their own way (and these people make most of the population). That's why in the modern army, you are mentally trained for that. People know the difficulty of shooting to kill, and train others to deal with that dilemma better. Kinda like explained in the video. The consequences he mentioned are the consequences of war, and would always anyway - the soldier who was mentally prepared for that just lives to suffer these consequences. And again, depends on the person, some would need mental help and some wouldn't. And in the past, as already mentioned, those who did that mostly loved doing that and were good at it.
Keep in mind that people you live around also effect you. It's the atmosphere. Thus you will share a lot with the people you live with, which further explains the written above.
