Atheist as synonym for evil (idea)

It was a big part of the demonization of "godless Communism", despite the fact that there was no such thing as a communist country, only communist parties.

Again, as stated in other writeups, "good" is defined as emanating from God; "evil" is that which is Not Good. If someone is equating "atheist" with "evil", that's their rationale. See morals and atheism for the morality thing; the catch-as-catch-can morality of secular religions doesn't cut it - look outside your window.