Grossman himself admitted
SLAM’s findings were controversial:
Marshall's findings have been somewhat controversial. Faced with scholarly concern about a researcher's methodology and conclusions, the scientific method involves replicating the research. In Marshall's case, every available, parallel, scholarly study validates his basic findings. Ardant du Picq's surveys of French officers in the 1860s and his observations on ancient battles, Keegan and Holmes' numerous accounts of ineffectual firing throughout history, Richard Holmes' assessment of Argentine firing rates in the Falklands War, Paddy Griffith's data on the extraordinarily low killing rate among Napoleonic and American Civil War regiments, the British Army's laser reenactments of historical battles, the FBI's studies of nonfiring rates among law enforcement officers in the 1950s and 1960s, and countless other individual and anecdotal observations all confirm Marshall's fundamental conclusion that man is not, by nature, a killer.
I don’t fully accept his explanation, but it’s clear that men in combat do not behave like wargame units.