"Deviant behavior by members of our group is perceived as more disturbing, and produces stronger retaliation"
Lt. Colonel Dave Grossman's "On Killing: The Psychological Cost of Learning to Kill in War and Society" is a fascinating work. It's required reading for much of the American military officers and law enforcement personnel. There's many counter-intuitive points in there, including that the vast majority (approximately 80%) of soldiers during the American Civil War and World War II never actually fired with the intent of hitting the enemy.
This paragraph stood out to me -
[In Dr. Jerome Frank's] Sanity and Survival in the Nuclear Age, […] he points out that civil wars are usually more bloody, prolonged, and unrestrained than other types of war. And Peter Watson, in War on the Mind, points out that "deviant behavior by members of our own group is perceived as more disturbing and produces stronger retaliation than that of others with whom we are less involved." We need only look at the intensity of aggression between different Christian factions in Europe across the centuries, or the infighting between the major Islamic sects in the Middle East, or the conflict between Leninist, Maoist, and Trotskyist Communists, or the horror in Rwanda and other African tribal battles, to confirm this fact.