Do you know the types of things that have been socially acceptable in cultures that didn't know God?
This statement could just as easily be re-phrased as, "Do you know the types of things that have been socially acceptable in cultures that
did know God?"
What percentage of the German population in the 30's and 40's were Christian, yet it seems the vast majority either approved of the Holocaust or turned a blind eye to it. Likewise (and I hate to keep hammering the same examples, but...) in the days of the witch trials people would be burned alive at the stake, and the good, God-fearing Christians of the town would all come out and watch it as a spectator sport.
My point is that history has shown that people do have a tendency to form a "mob mentality", and this can be done either in the name of religion, nationalism, or any other sort of "in group/outgroup" type of division. I agree that history records numerous examples of atrocious acts committed by atheistic regimes. However, I believe these were done as a result of a political ideology, which at times becomes so dogmatic that it becomes to its adherents almost like a religion.
I don't think you can blame atheism for these atrocities any more than you can blame Christianity as a whole for the atrocities committed in its name. Any belief system is going to have some followers who are "bad apples", but it's unfair for either side to paint the other using a broad brush in this way.
So, while you can point to atheistic Communist regimes who certainly have abused their power, you can also point to numerous examples of Christian leaders who have done the same in the name of God. Likewise, you can look at many modern-day European and Scandinavian countries that are secular governments with a large proportion of atheists in the population, yet those nations manage to live quite peacefully. Norway, Denmark, and Sweden are among the highest atheist populations in the world (they are about 70-75% non-believers), but when was the last time you saw any of these nations causing any trouble on the world stage?
I stand by my earlier statement that a world without religion could potentially (note I use the word
potentially, not
certainly) be a more peaceful world. This is strictly my opinion, but it seems that most large-scale conflicts in history have been the result of "ingroup" vs. "outgroup" delineations. It also seems like the biggest causes of these delineations are almost always either religion or nationalism. Without religion, we would knock out one of the two major causes of global conflict. Think how much easier it would be to resolve the Middle East crisis if there weren't deeply ingrained religious issues at the heart of the conflict.
I'm not naive enough to think mankind wouldn't still find other things to wage war over, but I do think reaching a lasting peace would be more feasible in a world without major religious differences dividing us.