All troops, when they occupy and battle insurgent forces, as in Iraq, or Gaza or Vietnam, are placed in "atrocity producing situations."
In this environment, surrounded by a hostile population, simple acts such as going to a store to buy a can of Coke means you can be killed. This constant fear and stress pushes troops to view everyone around them as the enemy. This hostility is compounded when the enemy, as in Iraq, is elusive, shadowy and hard to find.
The rage soldiers feel after a roadside bomb explodes, killing or maiming their comrades, is one that is easily directed over time to innocent civilians who are seen to support the insurgents. It is a short psychological leap, but a massive moral leap.
It is a leap from killing -- the shooting of someone who has the capacity to do you harm -- to murder -- the deadly assault against someone who cannot harm you. The war in Iraq is now primarily about murder. There is very little killing.
After four years of war, American Marines and soldiers have become socialized to atrocity.