Were any of these "atheist" regimes in your "test group" not totalitarian dictatorships?

You seem to be assigning causation for brutality here to the lack of belief in a deity, so firstly please evidence it was atheism, and not totalitarianism, that was the cause.

Several tests we could use here.

1. Are there totalitarian regimes that are theistic, and do these have a better record on human rights? Think Nazi Germany, ISIS, the Japanese fascists serving an Emperor deity, or Iran, Iraq, Saudi Arabia for example.
2. Cite any atheist doctrine or dogma that endorse or encourages brutality or the disregarding of human rights? Compared to say the sadistic brutality taught in both bible and Koran.
3. Are there any democratic atheist societies whose human rights record we can compare, for example Sweden, Denmark or Japan perhaps.

Your blatant selection bias is pretty obvious here. That aside, this tired old theistic cliche is fallacious of course, the fallacy is called post hoc ergo proper hoc. In English after this therefore because of this. In which selection bias is used to cite a reason you're biased against as the cause for an event coming after it, whilst ignoring all other potential causes. Theist reverse this process for causes they're biased in favour of, like prayer for example.

Peter Hitchens wrote an entire book based on this fallacy. He blamed atheism for the misery in the former Soviet Union, as if, like you, he'd failed to notice the fact it was a brutal dictatorship as well, where the citizens had no legal rights, exactly of course as was the case before, under the Christian Tsars, who were equally if not more barbaric in their utter disregard for the rights of ordinary Russians.

Like your post, Peter's book was complete nonsense. Intolerance can be pernicious, but there is no evidence at all that lacking a belief in a deity breeds intolerance. Quite the opposite can be evidenced in fact, in many democratic secular societies in the developed west.