Irrationality and murderousness are not necessarily limited to the religious mindset.
Not because they were atheist, but because they were totalitarian. And the figures are not exactly correct. The Nazis (21 million murdered) Chaing Kai-shek Nationalist Regime (10 million murdered) Japan (6 million murdered) Turkey (2 million murdered) Poland's ethnic cleansing (1.6 million murdered) and at least a million each -- Mexico and Feudal Russia, were not atheist at all and these account for 57 million murders by governments. (The actual total figure is more like 169 million, so your 100 million figure is correct for officially atheistic regimes, meaning they were not alone in their enterprise, just the most efficient.)
Irrationality and murderousness are not necessarily limited to the religious mindset.
But certainly religion is no protection from them. The cause of mass murders and oppression by government is ideology, specifically any ideology that believes men have the right to use unlimited force and coercion to impose their ideology on others. Except for those remnants of Marxism, Atheism today rejects all use of physical coecion in relations between men, and are usually the most outspoken about protecting freedom of religion. It is only in religion today that the belief one has a right to use force, especially government force, to impose beliefs (or practices based on beliefs) on others is alive and well, and there is plenty of that in the good old USA.
(I'm only comparing Atheism to actual Religions here. There is plenty of collectivist statist ideology in the world that is a-religious, such as American liberals and almost the entire leftist educational community, from top to bottom.) Hank