I know why Richard Dawkins didn't have much of an answer for Bill O'Reilly on his so-called "interview." It's because Dawkins has the same problem most atheists have with this statement.
They share the same belief: Statism.
Not that Hitler was an atheist. We've gone over that crap. Hitler was catholic, Hitler hated atheists, etc etc. But Hitler believed in Germany. Hitler believed that the State implied moral obligation. So do many, many atheists, who believe religiously in Democracy.
So how can we expect atheists to be able to answer this assertion about the worst tragedies of recent history? It wasn't atheism that murdered so many. It was belief that Might Makes Right. And as long as most atheists hold onto that belief they share with the Christians who point fingers at them, Christians will have the upper hand in the Game of Suffering. The answer to the immorality of Stalin, Hitler, the Inquisition and the Crusades?
The moral argument.
It's not an answer that favors a Movement, friends, not even atheism. It can't, ever. You want to oppose those atrocities? You have to be anarchist. If you favor monopoly of violence, especially from imaginary entities like God or the State, the Moral Razor slashes it to pieces. Atheism is just a first step to possibly understanding the material reality of morality. It cannot answer the tyranny of man.