I recently read a pop-history book called "Humankind: A Hopeful History" by Rutger Bregman; one of the central claims he makes is that humans have a fairly strong aversion to killing, especially the closer one must be to do so. He cites literature claiming that, for example, at the battles of Waterloo and the Somme, less than 1% of all casualties were by bayonet. A cursory Google search seems to support this psychological phenomenon: it's incredibly hard to stab or otherwise kill another human being with a blade.
Which brings me to the question: how did this psychological aversion to close-up killing play out in ancient and medieval warfare? Especially given the much lower level of training for your average premodern soldier which could desensitize them to killing. Were there just ridiculously low casualty rates as a result of this? How might a soldier with a bladed weapon get out of using it (since they can't shoot high, as one could with a ranged weapon, or double/triple/etc load it like muskets could be to avoid shooting)? If they did use bladed weapons to kill in warfare more easily than modern people, how did they overcome this psychological aversion without conditioning?
The answers I'm going to link are not quite the full picture, but they will provide a few more data points for you to chew on, and hopefully for other answerers to build off of. I will note that it's accepted among milhist circles that the majority of casualties in pre-gunpowder warfare come not from the battle itself (indeed, as you'll see, the winning side tends to take comparatively light casualties), but from the pursuit afterwards. Whatever aversion a human being may have for killing does not seem to be present when one is chasing down a running enemy.
As ever, more posts are always welcome, so if anyone else would like to write up, please go ahead and do so!