The Nazis? Stalin's purge? Stalin's forced famine? Pol Pot? WWI? WWII?
Rwanda? (the list is actually much longer) All just in the 20th century and nothing to do with religion.
I've heard your statement thrown around many times and it just ain't so. Money, power, greed, political ideology, tribalism, race, land, water, food....long list of why people kill people. Religion is sometimes used as a motivator, but the cause is human nature. You can make a case for science killing millions as well. It's still the humans behind the science.