I was looking up Grossman's On Killing for the statistics on how many soldiers in the U.S. Civil War actually fired at the opposing side (around 90% didn't fire at the enemy at all; it's a complex and interesting book).
One of the explanations for why is that there were two "filters" a person had to go through before able to do violence --
It is as though there were two filters that we have to go through to kill. The first filter is the forebrain. A hundred things can convince your forebrain to put a gun in your hand and go to a certain point: poverty, drugs, gangs, leaders, politics, and the social learning of violence in the media — which is magnified when you are from a broken home and searching for a role model.
But traditionally all these things have slammed into the resistance that a frightened, angry human being confronts in the midbrain. And except with sociopaths (who, by definition, do not have this resistance), the vast, vast majority of circumstances are not sufficient to overcome this midbrain safety net. But if you are conditioned to overcome these midbrain inhibitions, then you are a walking time bomb, a pseudosociopath, just waiting for the random factors of social interaction and forebrain rationalization to put you in the wrong place at the wrong time.