I love the old stories from antiquity which just happen to have men killing with impunity at no moral cost; in the bible there are passages where God commands to kill even infants; and there are, of course, all those horrific stories of what man can do to man from the Inquisition to the last crusades.
Today there's a stigma with killing. Watch almost any movie and even soldiers who kill show guilt. There's talk about the consequences of taking a person's life which burden the inner life. My supposition is those feelings presume a prohibition against killing. True or not, when did killing become codified into laws and when did -- what I'm assuming is a modern feeling -- of guilt start? Is there a certain culture or time period from which it fanned out, or does it happen spontaneously as cities/communities grow larger? I'm really interested in how we go from boasting about the number of people we killed to avoiding taking a life at any cost -- and I'm not asking solely of war.
I know this doesn't directly answer your question, but I don't think you are considering the multitude of examples of people showing remorse for their actions in stories from pretty much every time and place... or the ones from our own age where "bad guys" die continually and no one bats and eye about it. Greek tragedies have plenty of guilt and unglorified death to go around; not to mention that killing people in them can result in terrible punishments. You have Creon blaming himself for the death of Antigone and his son; Agave realising that she has killed her own son in The Bacchae... Not showing the feelings of soldiers and warriors in works of fiction can be a convention of genre or a stylistic choice-something that's not at all a universal trend. In the mythical-historical Chronicle of Japan (8th century), one of the enemies of Prince Shotoku is killed by the prince's troops, and is about to be dismembered. However, when the Prince learns that his dog was howling beside the corpse and guarded until it died of starvation, "the Court could not bear to hear of it for pity". The 12th century warriors in the Tale of Heike are constantly portrayed as mourning the death of their honourable enemies. One of the most famous chapters is the one where an old seasoned samurai has to kill a young kid who reminds him of his own son. Boasting about the people someone has killed and avoiding killing or feeling bad about it are in no aspect incompatible. Even in circumstances when a society approves taking the life of someone who (doesn't) belong to a particular group, what kind of people that includes is going to be codified.