Most humans, even when facing an aggressive enemy, display A Resistance to Killing:
Based on his postcombat interviews, Marshall concluded in his landmark book, Men Against Fire, that only 15 to 20% of the individual riflemen in World War II fired their weapons at an exposed enemy soldier. Specialized weapons, such as a flame-thrower, usually were fired. Crew-served weapons, such as a machine gun, almost always were fired. And firing would increase greatly if a nearby leader demanded that the soldier fire. But when left to their own devices, the great majority of individual combatants throughout history appear to have been unable or unwilling to kill.
In response to this research, the Army replaced bull’s-eye targets with realistic, man-shaped, pop-up targets that fall when hit:
The application and perfection of these basic conditioning techniques increased the rate of fire from near 20% in World War II to approximately 55% in Korea and around 95% in Vietnam.
Other factors in Overcoming the Resistance to Killing include proximity and respect of authority (e.g., your commanding officer grabs you, points at a target, and demands that you shoot him), physical and emotional distance to target (e.g., it’s easier to kill indirectly via artillery), and group absolution (e.g., being one member of a firing squad).
The continuous warfare of the 20th century led to mass psychiatric casualties — shell shock. From Psychiatric Casualties in War:
Swank and Marchand’s World War II study of US Army combatants on the beaches of Normandy found that after 60 days of continuous combat, 98% of the surviving soldiers had become psychiatric casualties. And the remaining 2% were identified as “aggressive psychopathic personalities.”