This raises a question: so how did things work back in, say, the Middle Ages or during the Roman Empire when battle often consisted of folks fighting at very close range and thrusting a sword into the opponent's belly? Were there a lot of non-sword thrusters?
That's actually a myth. Stand-up killing in ancient warfare was pretty rare. Most deaths occurred
after one army had broken.
There are some other factors to consider, however. Firstly, by far the easiest way to condition someone to kill is to actually get them to kill. Gangs in some South American cities train their child-recruits up by getting them to kill pets they've been keeping for a time. It's very effective.
In pre-industrial times your average man probably had a lot of experience with killing animals he owned because if he wanted to eat something he had to kill it. Humans naturally apply human characteristics to animals we keep, and that includes animals we're keeping as food.
Other examples include the training undergone by Spartans in ancient Greece - as a final part of their training they were sent out and required to kill helots for food. That's an excellent source of conditioning.
Atrocity, likewise, is a peer enforced conditioning process that uses
actual killing.
So it can be expected that a much larger percentage of soldiers in a historic army would have already killed and been involved in killing, than in a modern army.
Another factor is the place of the warrior in society. In historic times, the warrior was the penultimate figure in society, and an ability to kill in battle was seen as the highest of achievements. Society exalted and praised their killers.
Acceptance of the social group is vital for protecting the psychology of killers. In contrast in modern society soldiers are generally not held in such high regard, and in particular soldiers are not held in high regard for being killers.
Even WW2 veterans, who were universally praised as heroes, found that society didn't actually want to know about the details of war, and didn't want to know about killing.
Getting back to the dynamics of warfare in those times:
A standard battle would involve forces arrayed against each other for a time, until one force attacked the other. Either the defending force would break, or the defending force would hold in which case the attack force would usually break. It was usually that quick.
Once one side breaks, the rules all change. Just as it's easier to kill if you're looking through a sniper scope or dropping bombs on a city from 10,000ft, it's easier to cut down a fleeing soldier from behind and move on than it is to stand eye to eye with a man, kill him, and watch him die.
