What enables killing?
Here's a concise summary:

Demands of Authority
- Proximity of the obedience-demanding authority figure to the subject
- Subject's subjective respect for the obedience-demanding authority figure
- Intensity of the obedience-demanding authority figure's demands of killing behavior
- Legitimacy of the obedience-demanding authority figure's authority and demands

Group Absolution
- 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
1. Physical distance between the killer and the victim
2. Emotional distance between the killer and the victim, including:
- 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 sight, or some other kind of mechanical buffer

The Shalit Factors
Israeli military psychology has developed a model revolving around the nature of the victim, which I have incorporated into this model. This model considers the tactical circumstances associated with:
1. Relevance and effectiveness of available strategies for killing the victim
2. Relevance of the victim as a threat to the killer and his tactical situation
3. ''Payoff of the killer's action in terms of
- Killer's gain
- Enemy's loss

The Predisposition of the Killer
- Training/conditioning of the soldier (Marshall's contributions to the U.S. Army's training program increased the firing rate of the individual infantryman from 15 to 20 percent in World War II to 55 percent in Korea and nearly 90 to 95 percent in Vietnam.)
- Recent experiences of the soldier (For example, having a friend or relative killed by the enemy has been strongly linked with killing behavior on the battlefield.)
The temperament that predisposes a soldier to killing behavior is one of the most difficult areas to research. However, Swank and Marchand did propose the existence of 2 percent of combat soldiers who are predisposed to be "aggressive psychopaths" and who apparently do not experience the trauma commonly associated with killing behavior. These findings have been tentatively confirmed by other observers and by USAF figures concerning aggressive killing behavior among fighter pilots.

Now, at "distance from the victim" Grossman isn't precise because dehumanizing is true all the other options as he states in his book elsewhere. For example artillerymen aren't killing people, they just shooting at coordinates or proletarian revolutionaries aren't killing people but the opressor bourgeoisie.