In 1996 Lt. Col. Dave Grossman authored “On Killing,” a seminal study on “the psychological cost of learning to killing in war and society.” In it, Grossman documents the unheralded history of man’s inherent resistance to taking fellow human life. This history is so concealed that some, such as philosopher-psychologist Peter Martin have called it “a massive unconscious cover-up.”1 Indeed, despite the near universal glorification of combat across human society, the threads of this hidden truth are everywhere. For instance, Grossman recites a well-documented study of Prussian infantry accuracy...