HolofernesesHead, things like Stalinism and the Chinese Cultural Revolution, to name two religionless regimes, operated as Personality Cults, so ran along the very same lines as a religion, with people being brainwashed into believing that their Infallible Leader (ie Godhead) was THE only form of truth and all actions taken in His name would lead to glory, etc. So it is disingenuous and also totally incorrect to say that regimes that were set up devoid of religion set out to achieve and/or carried out certain heinous deeds in the name of Atheism. Atheism was not behind the deeds carried out. Bad things happened to be carried out by regimes that were irreligious. This is not the same as Uganda imposing the death penalty for homosexuality based on Biblical scripture, for example.
Furthermore, such regimes were also not created to promote Atheism, rather, it was a tool used as a social leveller and to guarantee full and singular devotion to the regime, rather than outside agencies such as deities.
In other words, none of the crimes commited by such regimes was carried out because of or for Atheism. It's a popular misnomer and also a tiresomely popular piece of ignorant crap peddled by religious people basically trying to say "See? You can't criticise religion because Atheists can be nasty too". Which, even if they were correct, would be very silly.
But I doubt you will comprehend or accept any of this, if you genuinely believe that Christianity is not an organised, monolithic religion! Even the Church itself describes itself as an organised religion. BTW, from a theoretical standpoint, monolithic actually means something so deply ingained in society that it has seemingly been present forever, and, as such, is privileged and seen as innately true and incredibly hard to change, confront, etc. Heteronormativity (the assumption that everyone is naturally and automatically heterosexual, that heterosexuality is the true and correct sexuality, and the promotion of heterosexuality as right and superior over other sexualities, etc.) is monolithic, for example.
Oh, and what SGB said. As usual 