"Atheist governments have been responsible for some of the largest mass murders and genocides in history! Look at Stalin, and Mao, and Hitler! Atheism leads to terrible things!"
There are at least three mistakes with the above. One of them is an unforced error; it doesn't always come up, but it usually does. We'll call it "problem #0". To wit:
Hitler wasn't an atheist.
Seriously. Go look it up. He was a kind of neo-Pagan, semi-Deist, quasi-Christian who thought Jesus was an Aryan. He explicitly rejected evolution and believed that species were created by a "Creator" in fixed kinds.
On the other hand, Hitler's ideas found fertile soil in Germany because of several centuries of virulent, religiously-motivated anti-Semitism. (Go read that link. Examine Luther's prescriptions. Don't they seem just a little familiar?)
Even if Hitler wasn't an atheist, there's no doubt that Stalin and Mao and Pol Pot and so forth were atheists. And they very definitely did kill millions.
But wait - even if it were true that atheism inevitably led to terrible things (and it's not) - how would that argue in any way that atheism wasn't correct?
Just because a fact has unpleasant, or even horrifying, consequences doesn't mean it's not a fact! You can't say, "It would be upsetting if I had cancer, so therefore I don't have cancer." No, how pleasant or unpleasant a truth is has nothing to do with whether or not it's a truth.
Let me point out something that few people who level this charge seem to notice. Stalin, Mao, and Pol Pot were all atheists, true - but they were, every one, Communist atheists.
You might not think that's an important distinction, but let's try a parallel argument and see if it's convincing.
No one disputes that Islam is a form of monotheism with roots in the Hebrew Torah.
Many strains of Islam have practiced forced conversion at swordpoint. And many strains of Islam have praciced brutal subjugation of women.
Therefore all of Islam is guilty of forced conversion and subjugation of women.
In fact, Christianity and Judaism are also monotheistic, and based on the Hebrew Torah! All forms of Christianity and Judaism must lead to the same barbarisms as well!
I suspect you were able to spot some problems with the above... 'argument'. Not all Islamic scholars support forced conversion or tossing acid in unveiled women's faces. And Christianity and Judaism, while sharing some basic elements in common with Islam, are also rather different in lots of ways. Even if Christianity or Judaism were prone to such things, the above 'argument' wouldn't suffice to establish that.
If there can be such a wide range within monotheism, why can't there be at least as wide a range in atheism?
If I tell you 'that person has a religion', that doesn't tell you much about what they actually believe. That covers paganism, Hinduism, (many forms of) Buddhism, animism, etc. Even if I tell you, 'They are a monotheist', it still doesn't define their doctrine much. I mean, they could be Christian, Jewish, Muslim, Zoroastrian, Deist, etc. etc.
Now, if I tell you someone is Christian, that does start telling you something about what they believe, it starts putting some limits and ranges on their beliefs.
Similarly, if I tell you that someone's an atheist, that tells you very little about what they actually beleive. If I tell you someone's a Communist atheist, or a Humanist atheist, or a "biophilosiphist", then maybe you can start forming some expectations about what they believe - and they would be different in all three cases.
I brought up humanism and "biophiloshopy" deliberately just now. Jonas Salk not only worked tirelessly to come up with the polio vaccine, he deliberately refused to patent it. He instead worked to make sure it would be as cheaply available as possible so it could help the maximum number of people possible. I myself argue for something other than violent conquest.
All "athiest" alone tells you is what someone doesn't believe. There's a huge range of beliefs that fit under the term 'atheism' - quite possibly more than fit under the term 'theism'.
As noted above, the case of Hitler shows that genocide is by no means limited to atheist states. There were two relevant difference between the 20th century and prior eras, though:
In other words, in the 20th century we had a much larger population and more efficient ways to kill that population. Both religous and non-religious societies engaged in genocide at that point. Not that this is surprising - read up on the Albigensian Crusade. If somehow, either side had been presented with a nuclear weapon, would they have hesitated to use it?
Some people blame the Holocaust and Communist genocides on the idea of evolution.
But remember how, above, I pointed out that Hitler rejected evolution? He also had that in common with the Communist states. Seriously - they also explicitly rejected neo-Darwinian evolution, embracing (and enforcing) Lysenkoism instead. The resulting crop failures when reality failed to match up to "worker's science" killed a huge fraction - possibly the majority - of the millions who died under those regimes.
Contrary to the repeated claim, the people under Hitler, Stalin, and Mao would have been better off if their leaders had accepted neo-Darwinian evolution.