Ibn Warraq gets this point dead wrong as we have witnessed the Carnage and bloodlust of Communists who were atheists but proclaimed themselves God. The cult of reason in the French revolution also has to be reconciled with the Great Terror and the Colonial wars of Napoleon . Napoleon does not belong in a conversation with Communism or Nazism. Nazism was the cult of man deciding what life is worthy and has far more in common with Communism . The idea of sending humans to concentration camps or anyone else for that matter is man elevating himself to God.
The view that polytheists or Pagans were more humane then the monotheists is baloney.
The Mongols were certainly not peaceful animists. Ancient Greece was colonial and often imposed its culture on others. African and American Indian tribes were a mixed bag of peaceful and warlike tribes. This blanket statement is way too broad and the historical record is mixed at best.