If Atheism in a broad sense is defined as the rejection of belief in the existence of deiTIES, then it's kinda tough to clearly label atheists as all good or all bad. There might be atheists who do not believe in the existence of a single almighty creator god and there might be atheists who do not believe in the existence of any kind of god in any non-human realms. So if we are to draw a Venn diagram of "Atheist" and "Buddhist", instead of seeing a complete separation between the 2 entities, there'll be overlapping areas. So, it's not a simple matter. In the case where there're atheists who do not believe in a single creator god NOR in any kind of gods in any realms, and yet they do not kill, steal, commit sexual misconduct, etc.. In that case, there should still be an overlapping "sila" area that is shared between Atheists and Buddhists. Even within the Buddhist circle of the diagram, taking closer look turns out to be that it's actually a sub Venn diagram of sub circles: there're those who only believes in the Elder's Way and dissing Mahayana and Vajrayana as fakes; and there're those who only believes in the Big Vehicle and dissing those Hinyanas as narrow-minded; yet they all believe in the 4NT, 8NP, 12DO, etc...So, it's not a simple matter I guess..