Greymare, check out this chart:
(Possibly) The Twenty (or so) Worst Things People Have Done to Each Other:.
How many can be described as religious? How many can be described as irreligious? How many involve institutionalized religion? How many involve
institutionalized atheism? Look at the body count numbers, and compare the counts that can be ascribed to institutionalized religion, to institutionalized atheism, and the irreligious ones. Look at the dates the atrocities took place, and compare the 20th century counts to the rest of history, then come to your own conclusion.