We'll hear this type of assertion in a number of forms:

  • Atheism is responsible for the deaths of millions of people
  • Atheism leads to genocide

The short answer is no, there's no causal chain from "I don't believe you" to genocide. The longer answer takes more time to explain.

Analysis

What is the Argument?

Depending on the construction of the argument, the purpose can range from simply trying to score points against atheists, to arguing that atheism is somehow intrinsically bad/wrong for civilization, and needs to be suppressed.

We'll address both, though the latter is probably more important discussion.

Scoring Points

I don't blame Christians for Hitler. I wouldn't say that Christianity produced Hitler. I do, however, bring up Hitler as an example of Christianity in action. This is specifically  in response to the assertion that one needs God to be moral, or that Christianity is needed to follow a moral life. Hitler is a counter point.

Absent those assertions, I'm fine with considering Hitler to be an aberration.

That is not typically how these arguments go, though. It is more like mudslinging between the U.S. Republicans and Democrats, where you've assigned yourself to one tribe or another, and you're grasping at anything that can be used as ammunition.

It is reminiscent of the arguments evolutionists get, that try to argue that evolution leads to racism/genocide. Even if we were to concede this, what's the point? That we should pretend that evolution is not real? That we should engage in willful ignorance about reality? If you want to consider yourself a "truth seeker", you have to take the good with the bad. If we concede that Christianity leads to peace, and atheism to war, that does not mean that a god is real.

Killing in the Name of Atheism

Does atheism lead to killing? Did atheism cause Stalin and Mao?

There's a couple of different angles we can examine this question. Speaking of one-liners, the NonProphets Podcast[1], once said something that stuck with me:
What does not believing in unicorns make you do?

In our case, what is the logical chain from not believing in any gods, to "hey anyone want to go murdering this Friday?"

Guilty by Association

On a very simplistic level, Stalin lead a murderous regime while being an atheist. That is asserted as an example of atheists killing people.

The problem here is, the association does not establish causation. We could probably draw a stronger connection between Stalin and being male (most dictators are male), yet the theists aren't arguing that it is wrong to be male, or that being male leads to being a genocidal maniac. We could also draw an association between Christianity, and a myriad of bad people in the last century. For some reason, they'll provide various reasons why these people aren't "real" Christians, at the same time that atheism merely being associated with Stalin is sufficient for atheism to shoulder the full responsibility of his actions.

It is true that many religious people were killed in Soviet Russia for the sake of promoting atheism. More on this later.

A Failure to Prevent

It is no secret that many theists have difficulty understanding how one can be moral without believing in a god. In some conversations, the theist will confess that they'd go on a murdering/pillaging/raping spree if he/she found out that there's no God. Mostly I suspect they'll assert this because they know we won't discover such a thing, and that it is the logical progression of their view of morality, and they are sticking to their guns.

With that in mind, it seems straight forward to think of atheists are morally unhinged. What is restricting them from a life of crime? It just makes sense, within that framing, that atheism leads to someone like Stalin being a murderer.

Of course, atheists would disagree. Penn Jillette is quoted as saying[2]:
The question I get asked by religious people all the time is, without God, what’s to stop me from raping all I want? And my answer is: I do rape all I want. And the amount I want is zero. And I do murder all I want, and the amount I want is zero. The fact that these people think that if they didn’t have this person watching over them that they would go on killing, raping rampages is the most self-damning thing I can imagine. I don’t want to do that. Right now, without any god, I don’t want to jump across this table and strangle you. I have no desire to strangle you. I have no desire to flip you over and rape you. You know what I mean?

I care about my fellow humans, and humanity in general. If I want to live peacefully, and without fear, my best bet is to recruit some neighbors as allies, and establish that world. That does not require any gods, and more often than not, is better accomplished without notions of a god derailing it.

 

Causation

We can abstract this discussion out - does atheism cause anything? At all?

Jillette's point is quite important, because it is core to the assertion many theists try to present - that a failure to prevent constitutes a cause.

Suppose that I wanted to swim in a lake, that is otherwise off limits due to it being a city water supply, and has a $1500 fine for swimming in it. That imposed fine may prevent me from actually swimming in the lake. That reasoning is fine... the rule drastically reduces the chances that I'm going to cause harm to the water supply. Similarly, if there's a God threatening me with Hell if I were to steal/murder/rape, my probability of doing so would decrease.

The problem theists make is that they try to reverse it: If believing in God decreases the chances of a person murdering (that is debatable in itself), then not believing in God increases the chances of a person murdering.

No, it does not. This has everything to do with the starting conditions.

If I wanted to swim in that lake, the fine may dissuade me from that. If I didn't want to swim in the lake, not having a fine does not make me want to swim in the lake. That desire must exist before the rules are applied (or not). Maybe I heard there's treasure below the lake... or that the water has healing properties. Some positive compelling reason must exist to compel me to take the action in the first place.

"I don't believe you" is not a positive compelling reason... for anything.

Setting aside the Bible's condoning of slavery, it does not have any command to not own slaves. There's no rule or commandment. At best, it has specific orders for specific groups (or individuals) to free their slaves, and that is it. Does the fact that the Bible failing to prevent people from owning slaves, in general, mean that the Bible caused slavery? Christians wouldn't accept that argument.

Stalin and Hitler

In many ways, Stalin and Hitler are two sides of the same coin. They were both dictators in pursuit of political power. The problem for such a career path is that religion is also often a competing political power. The dictators can either co-opt it, or stamp it out.

Indications are that Hitler was, at best, a mild believer[3], and he largely adopted Christianity for his own purposes (even if the subordinates believed). Stalin chose the latter option, imposing a largely atheistic government. Atheism, like Christianity, was a means to an end. This is evidenced by the fact that Stalin resurrected the Russian Orthodox Church specifically to grow patriotic support for the war[4].

Generally, one of Soviet Russia's goals was to spread atheism as official doctrine, but it raises an interesting question - do we have the cause and effect reversed? Did atheism cause Stalin, or did Stalin cause atheism?

Doctrine and Dogma

Atheism has no doctrine, dogma, tenets or rules, at it is core. The theists assert that one or more gods exist, and the atheists don't believe the claim. That is not to say that we can't have atheistic dogmas or doctrines (for example, Marxist philosophies), but that does not mean that atheism is dogmatic. That can't be achieved without people like Stalin adding additional elements.

Even considering Hitler, who did start with a religious doctrine, he too added in additional elements, but that does not mean that Christianity has those Nazi elements intrinsically.

Unlike basic atheism, basic Christianity does have tenets and doctrine. The Bible, often touted as a moral guide for society, has rules ordering for the stoning of unruly children[5], not suffering a witch to live[6], and slaves obeying their masters[7]. The Bible, and thus, Christianity, can be a causal factor, because it has positive compelling commands and rules, and followers who are raised and instructed to obey it.

"I don't believe in any gods", or even "I believe absolutely that there are no gods" is neither a rule nor a command.

At the end of the day

If we were to take the world's murderous regimes, and sort them into the categories "theistic" and "non-theistic", we'd find mixed results. Instead, if we sort them into the categories "totalitarian" and "non-totalitarian", the murderous regimes and administrations would be heavily lopsided towards totalitarianism. (This is more fuzzy when we start considering whether the country/regime is killing people inside or outside its citizenry)

"I don't believe you" is not totalitarian or authoritarian. "I think that religions are most likely fairy tales, made up by people who fear death", while harsh, is not totalitarian.

"Thou shalt have no other gods before me", is.

In fact, if you're looking for a happy, healthy society, your best bet is to find a non-totalitarian secular one[8]:
Pressing questions include the reasons, whether theistic or non-theistic, that the exceptionally wealthy U.S. is so inefficient that it is experiencing a much higher degree of societal distress than are less religious, less wealthy prosperous democracies. Conversely, how do the latter achieve superior societal health while having little in the way of the religious values or institutions? There is evidence that within the U.S. strong disparities in religious belief versus acceptance of evolution are correlated with similarly varying rates of societal dysfunction, the strongly theistic, anti-evolution south and mid-west having markedly worse homicide, mortality, STD, youth pregnancy, marital and related problems than the northeast where societal conditions, secularization, and acceptance of evolution approach European norms.

Ultimately, atheism bears no responsibility for the action of atheists, any more than those who don't believe in fairies bear any responsibility for any action other people, who also don't believe, have taken.

With Stalin, atheism was not the problem - it was being a murderous totalitarian dictator.

If atheism had a book with rules and tenets, that all atheists are following, to at least some degree, one could begin to make that argument. That much is important, because it makes no sense to call one's self a "Christian", if nothing of the book is believed/followed.