Mao was an atheist
Mao Zedong was an atheist, there is no denying it. But his atheism was, as with any other atheist, not the cause of his immoral actions. Mao was one of the founders of Communism in China and his legacy is ever-evolving, although he caused the deaths of millions, Mao is a very controversial character in Chinese history, he is said to have been one of the most influential people in Chinese history by some and is claimed to have made China a superpower. He didn't use atheism for his benefit to further destructive plans, he just happened to be an atheist. Without justification that atheism somehow was involved in Mao's actions, the argument commits the association fallacy.
An argument of 'Person X was an atheist' carries no weight whatsoever. A use of this sort of argument is by theists who want to justify their beliefs and make atheism to be similar if not worse i.e. tu quoque. This sort of theist isn't as bad as some theists out there, they realise that their Bible contains some pretty horrific material  and are not merely trying to rationalise it.