Caiphus
*error*
When you look into killology, it was hard enough to get soldiers to shoot at the enemy only - about two percent of were actually aiming at the enemy during WW2 rather than posturing. At least until they really changed things in regards to military training pre Vietnam. One of the keys is shared responsibility for the killing or deferred responsibility to a commanding officer. Add in the fact that there have frequently been armed people who have decided not to act at the sight of mass shootings because they didn't want to risk confusing first respondents.