I would argue that Socialism and Communism (formally atheist) are the root of the problems around the world. Certainly the greatest body count racked up by the millions.
Actually, this is false. The death toll attributed to religion is in the hundreds of millions. 100m is attributed to Christianity alone [in the same sense that deaths are attributed to communism and socialism, where deaths are caused by actions from leaders of a particular ideology or religion whether they be directly attributed to the same prevailing ethos or not]. But to be fair, religion had a ~4,000 year head start on socialism and communism.