S.L.A. Marshall was a fraud, and “On Killing” should be read with the knowledge it was written by a fraud.
https://en.wikipedia.org/wiki/S.L.A._Marshall
Some veterans and historians have cast doubt on Marshall's research methods.[15] Professor Roger J. Spiller (Deputy Director of the Combat Studies Institute, US Army Command and General Staff College) argues in his 1988 article, "S. L. A. Marshall and the Ratio of Fire" (RUSI Journal, Winter 1988, pages 63–71), that Marshall had not actually conducted the research upon which he based his ratio-of-fire theory. "The 'systematic collection of data' appears to have been an invention."[16] This revelation has called into question the authenticity of some of Marshall's other books and has lent academic weight to doubts about his integrity that had been raised in military circles even decades earlier.[17]
In his 1989 memoir, About Face, David H. Hackworth described his initial elation at an assignment with a man he idolized, and how that elation turned to disillusion after seeing Marshall's character and methods firsthand. Hackworth described Marshall as a "voyeur warrior", for whom "the truth never got in the way of a good story", and went so far as to say, "Veterans of many of the actions he 'documented' in his books have complained bitterly over the years of his inaccuracy or blatant bias".[18][19]