I just used StumbleUpon and found this page: http://www.godandscience.org/apologetics/atrocities.html.
The first chart was interesting, showing how many people have been killed for other reasons than religion. Most of those were due to land and power struggles, invasions, and corrupt or inept governments. However, every one of those countries or groups had religion of some sort (even the Mongols, who were very open to all religions), so we can't be for sure as to whether religion was used to gather support for those actions (as in "God has told me we are to invade so-and-so country". Religion wasn't a factor in the Iraq War either, but GW Bush claimed God told him to do it.)
It was the second chart, the one with the 20th century figures, that caught my attention. This chart has "Atheist?" as a column header, denoting whether each country was atheist or not. I noticed something right away. The atheist countries the chart states are USSR/Russia, China, North Korea, Vietnam, Cambodia, Yugoslavia, and Poland. Let's take a look at each one:
What are we to get from this? The author of this article states "If one examines the nature of the regimes that committed these atrocities (even the religious ones), the key factor is absolute power." True, but the difference is that the atheist states that committed the atrocities did NOT do them to spread atheism. They did them to gain absolute power and to maximize productivity. Just because the states were officially atheist doesn't mean that they were forcing people to convert to atheism. They saw religion as a threat to productivity, society, and progress and people who were religious threatened those things. True, they were completely wrong for killing people, but atheism was not the cause, simply the tool. If you look at the Crusades and the Inquisition, those events were SPECIFICALLY to convert groups of people in order to gain the numbers advantage, and thus power, or kill them. Religion was the cause AND the tool.
As far as today's terrorism, both foreign and domestic, people are using their interpretations of religion to justify their actions. As the author states in this article, "Today, some people kill abortionists in the name of God. Are these people unwilling pawns of religion or using religion to justify their own evil agendas?" They aren't unwilling. They are more than happy to blow up an abortion clinic because they think they are doing God's work. If there were no religion, the bomber would have no reason to bomb the clinic. If there were no religion, the idea that abortion was condemned by a sky deity and the doctor and the patient were going to hell wouldn't exist. And if blowing up a clinic were his evil agenda, then the man is crazy and lacking morals and should be punished as such. There are completely sane, intelligent, stable people who have blown up clinics and have used religion as the reason. If belief is strong enough, one will do anything for that belief regardless of whether it's rational, logical, or moral. Look at the 9/11 terrorists. They were all very intelligent, stable people. However, their belief in their religion got to the point where it consumed their logical, moral centers and they acted upon their beliefs.
The author of this article was trying to illustrate that atheistic groups have committed mass murder and tried to deflect the notion that religion is the most used reason to commit these mass murders. Instead, what he showed us is that countries used forced atheism to improve their countries (China and Russia were feudal agricultural countries before communism built them into industrial superpowers) and religion killed people simply for not changing their mind about which god they worshiped. Religion killed to spread their influence and gain power and wealth because God told them to. Atheism turned largely farm countries into massively productive, culturally progressive superpowers that have lent their hand in improving technology, science, medicine, the arts, and the world economy. The ones to blame for all those people dying in communist countries are the leaders. Stalin, Mao, Minh, Pol Pot. All power hungry, maniacal dictators who did not handle religious people as they should've. In all, atheism is not to blame.