I'm aware of the book you mentioned,
"On Killing: The Psychological Cost of Learning to Kill in War and Society", by David Grossman and
"Men against Fire" by Brigadier General Samuel Lyman Atwood Marshall. I think this is all a 'myth' and pure crap. Extraordinary claims require extraordinary evidence, and there is no evidence to support the claim.
If you are interested, Robert Engen wrote a very incisive article on the subject in the Canadian Military Journal, and wrote his Masters thesis on the subject. Engen found (and has the evidence to prove) that - for Canadians, at the very least - did not have this problem. Based on primary sources (written post-combat interviews with Canadian officers), he found exactly the opposite of what Marshall and Grossman claim.
Link:
http://qspace.library.queensu.ca/bitstream/1974/1081/1/Engen_Robert_C_200803_MA.pdf
It is not rocket science. If you and your buddies don't fire back at the enemy firing at you, you and your buddies get killed. Think of the
Battle of Gettysburg during the Civil War, would the Union have won had they not fired back at the Confederates?
Not only were American soldiers (and Marines) firing their weapons during World War II and Vietnam, in Vietnam American soldiers were killing 2nd Lieutenants (Platoon Leader) in the field - fragging. In some cases we have evidence and in others cases we cannot prove it.
"Well, the LT was in my field of fire!" Other U.S
. soldiers would use an AK-47 for just that purpose.
- On 21 April 1969, a grenade was thrown into the company office of K Company, 9th Marines, at Quang Tri Combat Base; 1st Lieutenant Robert Rohweller died of wounds he received in the explosion. Private Reginald Smith pleaded guilty to the premeditated murder of Rohweller and was sentenced to 40 years' imprisonment.
- On 15 March 1971, a grenade tossed into an officer billet at Bien Hoa Army Airfield killed Lieutenants Thomas Dellwo and Richard Harlan of the 1st Cavalry Division. Private Billy Smith was charged with killing the officers but was acquitted.