Therein lies the paradox. We see history, as you have posted, and conclude what you have. OTOH how do you explain things like this, of the muskets recovered on the battlefield, of fallen soldiers during the Civil War, most were loaded 5 or 6 times but never fired. The rifles of soldiers killed in the Battle of the Bulge were found to have NOT BEEN FIRED. I can give example after example of the fact that Joe Six Pack is NOT a killer. He just ain’t.
Most people don't kill as a daily occurrence. It's possible to condition people to kill without hesitation. Why do you think ISIS starts conditioning their kids to be acclimated to violence and killing?
And I can cite a thousand where he is not only a killer, he relishes it.
Maybe the rifleman didn’t fire his weapon as often as we expected him to, but the cannoneers and cavalry and — in latter days — the bombers and strafers didn’t seem to have much of a problem dealing out death wholesale. Look at Dresden or Hiroshima.
Technology has allowed us to be more detached from the slaughter, but that just makes it easier for us to be murderous.
Sorry, man, but I’m not buying it. Even Joe 6 is constrained only by a thin veil of civilization. Underneath is fang and claw.