What do you mean, “Conditioned to kill.”?
A huge number of Japanese and Germans died in WWII by stray bullets from unfired weapons if the study you refer to is illustrative of American combat units in general.
I have a problem with studies undertaken by liberals, psychologists, and and the anti-firearms camp.
Since the get go, Armies have have had trouble and have to this day used all methods to get one human to kill another.
Even citizens coming out of mind controlled states like Imperial Japan, Hitler's Germany, or Stalin's Russia have had to use execution as a motivation.