I decided to try and dig into the facts as best I'm able to get a handle on this claim. Who is responsible for creating the most corpses? I'm aware of many US backed atrocities that people don't talk about a whole lot. I bet if we were to tally them we might be surprised at the facts.
I found a great resource here from someone named Matthew White. He compiles the various death estimates from what appear to be credible sources and attempts to make a judgment about the totals on the basis of that information. The link I provided focuses on the top 30 atrocities of the 20th century. Digging deeper into the website provides additional information, including in the case of WWII a break down of who is responsible for what amount of the death.
What I've done is attempted to break the totals down by the religion of the responsible party. This is a little subjective and I'm certainly open to modification, but here's how I approached it. In the case where I know who an aggressor is I attribute all death to the aggressor nation. So for instance the Soviet Union invaded Afghanistan and Muslim Afghans retaliated and killed Soviets. I'm attributing all death to the Soviets in that case. If on the other hand I don't know how to identify the aggressor I partition the death between the agents to the conflict. If a pretty good majority of all agents belong to a particular religion I attribute all death to the majority religion. If it's nearly 50/50 I likewise split the toll.
Also death includes death due to starvation. Mao and Stalin didn't necessarily put a bullet in everyone's head, but they enacted policies that lead directly to death and so they are responsible for that.
In the case of the Iran-Iraq war I split the death between Christians and Muslims because Saddam was installed by the US and both sides were provided weapons from the US. So the foot soldiers were Muslim, but the backing was Christian.
I've added a couple of additional atrocities that weren't in White's list. I use his figure for Iraqi sanctions, but it wasn't on his list possibly because he was tallying what he calls "bloodlettings." There may be other incidents that belong and I'll add them as I come to know them.
He also didn't list the Iraq/Afghan war, so I've added that. My source is here. I've also included the WTC bombings simply because they are prominent.
Numbers are in thousands.
|1886-1908||Congo Free State||8000|
|1917-1922||Russian Civil War||9000|
|1917-1937||China Warlord Era||4000|
|1936-1939||Spanish Civil War||365|
|1945-1949||Chinese Civil War||2500|
|1945-1954||First Indochina war||400|
|1948-1999||N Korean Communist Regime||400|
|1949-1976||Mao Zedong's China||40000|
|1955-1972||Sudanese Civil War||500|
|1962-1992||Ethiopian Civil War||1400|
|1965-1966||Indonesian Communist Massacres||225||225|
|1967-1970||Nigeria: Biafran Revolt||500||500|
|1971||Indo Pakistani War||625||625|
|1975-1994||Angolan Civil War||300||300|
|1976-1979||Pol Pot's Reign||1650|
|1976-1992||Mozambique Civil War||800|
|1980-1989||Afghanistan Soviet War||1500|
|1983-1999||Sudanese Civil War||1900|
|1998-1999||Congolese Civil War||1700|
|2001-present||Invasions of Iraq & Afghanistan||1036|
So let's notice something here. Yes, atheists are in the lead. But the facts kind of help us recognize that this argument is not very forceful. Is atheism obviously wrong due to the death of 83 million, but Christianity is not obviously wrong because they only killed 74 million? And I would also note that this time frame is convenient from a Christian perspective. If you go out another 25 years you can tack another 26 million on the heads of Christians due to famines induced in colonial India by the British. That's enough to give Christians the lead.
Also for the most part I'm sticking with Matthew White's top 30 list. I think as I look for additional smaller items I'll see much more religious death. All the dictators imposed throughout Latin America for the last 50 years will make a big difference. Other CIA adventures (Shah in Iran, terrorism in Cuba, backing of Indonesian invasion of East Timor under Ford and through the Clinton years, etc) would again add to both the Muslim and Christian totals.
What pops out to me though is the paltry amount of Islamic death. A mere 5 million? If this argument proves anything it proves the truth of Islam.