Ah, but Stalin and Hitler and Mao! Give me a break. Sure, they were atheists. But they did not kill because they were atheists. Hitler was a fanatical racist and Mao and Stalin fanatical communists, and they killed in the name of those fundamentalist philosophies.
Let's ignore Hitler for a second, because his religious beliefs are not at all clear. So let's focus on Stalin and Mao, who were both unambiguous about their nominal atheism.
I am going to one step further than The Economist, and argue (as I have in the comments on other blogs) that Stalin and Mao were not actually atheists in any useful sense of the word.
Atheism is usually defined as the lack of a belief in a god or gods. Okay, so how do we define the word "god"? Well, you could write a frikkin' book about that, but certainly some important elements would be infallibility, omnipotence/absolute control, etc. There exists another concept to which these attributes are integral: the totalitarian state.
Any useful definition of atheism -- or at least, of secularism -- has to reject the idea of a single uncompromising source of authority. Otherwise, the definition gets watered down to something almost meaningless, e.g. atheism is the lack of belief in Jesus, Buddah, Yahweh, Shiva, Brahma, Zeus, Thor, etc. In order for the definition to be useful, there must be a functional definition of what beliefs atheism rejects, rather than just a laundry list of deities.
In a totalitarianist dictatorship, the dictator is god, by any useful functional definition. He1 is all-powerful. He cannot be questioned. His command(ment)s must be obeyed. In the case of post-Mao China, there was not a single dictator but it was still a totalitarianist state, and in that case the bureaucracy fills the role of god.
Still don't believe me? North Korea has even had the good grace to formally codify the country's totalitarian ideals as a religion.
If you are skeptical about this expansive definition of god(s), consider the Heaven's Gate cultists. Certainly they did not appear to believe in god(s) in the traditional sense, but it would be laughable to refer to them as a "secular" or "atheist" group. Clearly they were not. To me, it is equally clear that Stalinist Russia could not be reasonably labeled as secular or atheist, either.
But wait, you say, even if we accept that Stalinism and Maoism are not actually atheistic belief systems, that doesn't mean that Stalin or Mao couldn't be atheist! Well, sort of. We can ask whether Stalin really believed in his heart that he had the wisdom to command all of the Soviet Union, but not only is this question unanswerable, it is ultimately irrelevant.
It is almost a surety that some fraction of the Catholic priests who have been convicted of child molestation had internally abandoned their faith prior to their despicable actions... does this mean that the molestations were now committed by atheist priests, and that atheists must answer for their actions? No, of course not. The priests were part of the Catholic infrastructure, they self-identified as Catholics, so we have little choice but to say they are Catholics.
By the same token, it's possible -- maybe even likely -- that Stalin and Mao did not actually believe in their own infallibility. But to say that this means they weren't truly totalitarianists is about as meaningful as saying that maybe David Koresh didn't literally believe he was a prophet and therefore he wasn't a Branch Davidian. That's retarded. Stalin and Mao preached totalitarianism, therefore they were totalitarianists, not atheists. End of story.
In short: It is my contention that secular totalitarianism is an oxymoron. Totalitarianism by its very nature is non-secular, because it promotes ideals of infallibility and absolute obedience that are not compatible with a secular worldview. By definition, no totalitarian dictator can be rightly termed an atheist.
1I'm normally rather careful with pronouns, to either say "he or she", or if I am writing informally, the singular "they". However, I'm not aware of any female totalitarianist dictators, so I think I am safe in using the male pronoun here. Correct me if I'm wrong!