I think there is truth in the notion that it is psychologically easier to kill from a greater distance. But I think in terms of quantity of killing, with the exception of nuclear weapons, more has been done at close range. While probably exaggerated, accounts of Alexander's battles number tens of thousands killed in a single day. Take a two masses of people, arm them with sharp pointy things, give them a shield and a chest plate, smash them together and see how many of them die.
It's possible that in Napoleon's time the close-ranked soldiers tended to die more under artillery, and tactics have changed to spread soldiers out and make each artillery hit less effective. So tactics weigh into it quite a lot.
I don't remember where I read it, but I know that Napoleon had a devil of a time keeping his artillery shooting at the enemy infantry, where it did the most damage. The gunners had a tendency to aim at the other artillery, which resulted in a lot of wasted ammo and time.
I still think the bayonet charge that was used until very recently (even if it did outlive it's usefulness), held on for such a long time because it is part of our instinct -- a modified spear attack. If we're talking about "Human style" fighting, as compared to "tiger-style" or "praying mantis", then long-range warfare (i.e. beyond eyesight) is too far removed from the situation to really engage the "human style" of fighting.
I don't know . . . does pushing a button from a thousand miles away still give you that adrenalin dump that actually seeing the person you are considering killing gives? Are you still engaged at that point?