Ex-US Ranger Lieutenant Colonel (retired) Dave Grossman has done some excellent research on how people in wars come to kill other people. His site on this and on the related issue of preventing violence by veterans of wars who have been trained to kill is at:
His rigorously researched book is called ‘On Killing: The Psychological Cost of Learning to Kill in War and in Society. It is a book about the real origins of much combat post traumatic stress disorder. Prior to the 1950s, only about 20% of infantrymen would kill someone else, in close combat, even to save their own lives. Frustrated by this, militaries starting in the US, turned to behaviorist psychology and by training shooting reflexes, helped overcome human resistance to kill in basic infant training. This has had enormous human cost, when veterans return to civilian life and there was no associated de-programming, or understanding of the conditioning they had received by the soldiers concerned, leading to extensive substance abuse, violent anti-social behavior and large scale failure to adapt. He also indicts violent video games and TV movies for exacerbating this killing instinct.
The US Military are beginning to act on his findings. Meanwhile in the UK, for instance, a significant proportion (up to 10%) of the prison population are ex-military veterans and there is little understanding of this issue.
This is Dave briefing US soldiers on his findings and their implications. His argument is not about the need for nations to defend themselves or soldiers to have to kill. It is about how we train and then un-train them for this role: