How did soldiers in pre-gunpowder warfare mentally deal with the nature of battle?
Due to the nature frontline hand to hand combat and siege warfare, it must have been brutal beyond comprehension. How could a human being convince themselves to throw themselves into a meat grinder like that, or to be the first guy up a siege ladder? Or being first in an infantry charge, or any other situation where death is basically guaranteed? How do you watch a dozen guys in front of you die horribly and go "oh boy it's my turn, time to make a difference!"
Without meaning to dissuade further discussion, I think the main issue here is the false premise that pre-gunpowder close combat involved the systematic destruction of rank upon rank of warriors. This is not at all what we can glean from contemporary sources. Few men died in the actual engagement; the majority of casualties was inflicted when one side broke and ran. I wrote more about this here.