Well, the twentieth century also saw an exponential rise in killing efficiency but I agree with atheists—declared and de facto—must accept some responsibility. However, be careful, don’t focus on the danger of Communism and ignore its generic form: big government. Few people would accept “communism” these days but far too many countries are accepting socialism and, when the going gets tough, socialism can easily morph into communism in effect, if not in name.
As a non-believer, I accept absolutely no responsibility whatsoever in the murder, genocide, or oppression of any other human being, or group of human beings.
I am opposed to authoritarianism and murder in all of its forms, and no part of my belief systems supports anything of the sort, nor would I ever in any case need to resort to such methods to live by those beliefs.