On Killing by LtCol Dave Grossman, 2009, Excerpts
The effectiveness of modern conditioning techniques that enable killing in combat is irrefutable, and their impact on the modern battlefield is enormous. Three primary situational variables that influence or enable killing behavior are;  the demands of authority,  group absolution, and  the distance from the victim.
Demands of Authority
- Subject’s proximity to authority
- Subject’s respect for the authority
- Intensity of the authority
- Legitimacy of the authority
- Subject’s identification with the group
- Proximity of the group to the subject
- Intensity of the group’s support for the kill
- Number in the immediate group
- Legitimacy of the group
Total distance from the victim
- Physical distance
- Social distance, which considers the impact of a lifetime of viewing a particular class as less than human in a socially stratified environment.
- Cultural distance, which includes racial and ethnic differences that permit the killer to dehumanize the victim.
- Moral distance, which takes into consideration intense belief in moral superiority and vengeful actions.
- Mechanical distance, which includes the sterile video game unreality of killing through a TV screen, a thermal sight, a sniper site, or some other kind of mechanical buffer.