This is a frequent and tiresome ad hominem frequently resorted to by religious fundamentalists fast approaching total cognitive surrender, and I'd like to include a complete rebuttal here before I forget it.
To begin with I'm going to leave Hitler to one side and examine our other three lumninaries. What did Joseph Stalin, Pol Pot and Mao Tse Tung all have in common apart from their atheism? Anyone? They all had a fanatical devotion to collectivism.
In short all three subscribed to a dogmatic ideology that considered the wellbeing of the collective to be of greater import than the wellbeing of the individuals that made up the collective. Individuals percieved as threats to or targets of the collective were eliminated by exile, starvation or good old fashioned killing.
The problem with ideologies is that they inevitably centre around the notion that ideas are more important than people. Accepting that ideas are more important than people inevitably leads to tragedy and atrocity.
Atheism has no ideology. The central idea of atheism is a lack of belief in gods. In fact the only idea of atheism is a lack of belief in gods. Atheism can form a part of an ideology but it can never ben an ideology in itself because there's not enough to it.
Now as for our friend Adolf, first an aside:
Hitler was not an atheist. It takes some spectacular mental contortions to come to the conclusion that Hitler was motivated by or even entertained the notion of atheism. A self declared "Christian...fighter for Truth and Justics" who declared himself an agent of "the Almighty Creator" makes for a lousy atheist. It's entirely understandable why you would want to distance yourself from Hitler, but attempting to use his name to sully your opponents is simply dishonest.
So what did motivate Hitler? Well it wasn't collectivism. It was however a fanatical devotion to an ideology which valued ideas more than it valued people! Hitler's ideology of Nazism, largely of his own invention, was a mish-mash of misunderstanding the works of Neitzsche and Heidegger who in turn were busy misunderstanding the works of Charles Darwin.
Hitler's ideology was based on germanic and aryan supremacy and slavic and jewish inferiority rather than on political collectivism and focused on race war rather than class struggle, but it was still at heart an ideology which crushed its targets and opponents under foot.
Yet again we see that when ideas are considered to be more valuable than people it is inevitably followed by tragedy and atrocity.
These four great apparently secular tyrants all had the same thing in common. Where else do we see this phenomenon? Every time an alleged witch is burned, every time a homosexual is murdered, every time a woman is beaten or raped for not covering herself up, every time a person is convicted or persecuted for unislamic or unchristian behaviour, every time an apostate is executed or a blasphemer is censured or threatened we are seeing a demonstration of ideas being held as more important than people.
I believe that personal liberty and freedom of speech are ideas worth fighting for and worth defending precisely because these ideas put people first. They protect the individual from the scourge of ideology.
If we celebrate the ideology of liberty and free speech you can be sure that you are putting people ahead of ideas. Isn't that the way it should be?