The idea of understanding “why” people or groups commit atrocities is attractive, but unfortunately it can be hard to pinpoint a reason.

Excessive brutality in warfare has a long history. Few people realize that taking bodyparts of your enemy as trophies was common in WW2, american forces in the pacific did that a lot for example. The sheer hatred you may end up feeling for the enemy is enough to push you to do things you wouldn’t imagine, since brutalizing a corpse has a lot of meaning psychologically.

For what concerns “institutionalised” brutality, like in some comparable cases such as imperial japan’s treatment of China or the Belgian Congo, you have a combination of two factors:

1. An ideology centered around the fundamental inferiority of the enemy, which leads to dehumanization
2. A culture that promotes nationalism and militarism/militancy, which causes people to inflate the value of their “mission” and thus of the lengths they are willing to go to accomplish it
3. A military command structure that emphasizes local autonomy, meaning that crimes commited by individual groups of soldiers have a smaller chance to actually make it up the ranks and be tried, as the leading officer doesn’t necessarily need to report everything to their superiors.

I believe democratic Kampuchea sort of… adapted? those principles to a “communist” context. It was an agrarian movement that despised urbanites and urban life, due to a deep social divide. They were an extremely militant movement, and they were organized in local cadres who enjoyed a great level of autonomy (and thus impunity).