Yes, but then we're left with the question of why those levels of killing did not go on in lands that did embrace worship of a Creator. If the presence of atheism means nothing but the presence of industrialized weapons does, then why didn't the Christian nations slaughter all their inconvenient citizens? Why didn't we see death camps in those countries, or others where a firm religious mindset was prevalent?
Because by then religion didn't have much power over the governments of most of the world. Most Christian countries had converted to democracy, a secular check against the crusades that the churches may have wanted to launch.
While I am not religious, I do like the check-and-balance system between the secular and religious that most Western countries have.
"Yes, but then we're left with the question of why those levels of killing did not go on in lands that did embrace worship of a Creator."
There's a problem with that. Consider the Islamic countries. They certainly worship a creator, yet are quite warlike and destructive.
I fear that the mere belief in deities does not preclude a society from being warlike or even genocidal.
The key combination is 1)industrialization, giving the means to kill en masse (and the larger population of potential victims in the first place) and 2)total control.
The Christian church never really had the second, and hasn't been anywhere close to it in modern times (between internal fractionalization and competition with secular institutions). Radical Islam is dangerous because it does have total control of some nations, but fortunately lacks access to large numbers of victims who will go down without an effective fight -- if it did, do you doubt for one moment that it would establish death camps and consider them to be divinely blessed?