So Christianity, then, is judged by its adherents and not by its followers? Then I have news for atheists: Some of the most vile creatures on the face of this planet were atheists. You still want to use that equivalence?
that's a false equivalence; atheism is simply a lack of belief in gods, it makes no normative statements. there's no "atheist moral philosophy" that is centered around a lack of belief -- how could there be ethical / moral statements attached to that?
if you would like to compare christianity with moral philosophies, i would suggest perhaps utilitarianism, or hedonism.
I'm assuming you're referring to Stalin's purges in the Soviet Union, and Mao's similar activities in the PRC. Neither of those
monsters (see, i agree with your ethical implication) were primarily atheist -- it was never a governing, necessary trait for either of their policies / actions. At best it was an expression of their drive to consolidate all power within their own controlling hierarchy; neither could tolerate any sort of outside influence in their dictatorship. In the case of Stalin, the Russian Orthodox Church was closely associated with the Romanoff dynasty, and in the case of Mao, his primary consideration was the elimination of "foreign" or "superstitious" influence. Both considered religion to be an enemy, but for purely practical control considerations, not philosophical ones.
And if you're implying that the reverse is true, that religious states, in particular christian ones, are more ethical than those that are not, you're ignoring the historical record entirely, as well as current social trends. Religiosity does not correlate well with what are commonly considered positive social traits, even by the standards of the religious, and in fact, oftentimes is the opposite -- look at crime, drug addiction, teen pregnancy, etc. in the US and compare that to religiosity.
If christian religion was such a panacea, then I would think that the Bible Belt would be a correspondingly lovely place, instead of having some of the highest crime rates in the nation, with five out of the top ten slots in terms of overall crime.