After I posted about it yesterday, I did some further thinking and something occurred to me. If you are a theist who wants to make that argument, weak as it is, why bring Hitler into it at all? Someone is bound to point out that he wasn't actually an atheist. Making an obviously false assertion right up front isn't a good way to convince anyone.
Are the horrors of Stalin, Mao, Pol Pot & other communist regimes just not bad enough without adding Hitler? I mean, depending on how you calculate, Stalin & Mao each have Hitler beat in sheer numbers of people killed. So the next time you trot out the evil atheist regimes show how bad atheism is argument, why not just leave Hitler out of it? That way we atheists can focus on demolishing your main thesis.