Since atheists are often accused, unfairly of course, of causing the biggest genocides of the last century, I wrote a detailed article on how to respond to the charges. The strategy takes into account the fact that many religious people are not really aware of how cause and effect works. Nevertheless, the approach to this problem differs from individual to individual.
I have listed 6 possible answers that could be used, sometimes in combinations of two or more, to debate any religious zealots who are intent on redefining history with a lie. You can see my original post here: