If we didn't have religion, we would revert to tribalism as a substitute. Just look to Somalia to see what it would be like.
A very odd statement. Tribalism is inherently religious. The Somalis are very religious.
I think, in interpreting what many people here are saying, is that they think of religion in strictly western/levantine terms: a universal religion, absolutely true for all time and for all peoples, without exception. If that is "religion" then some "bad guys" have been religious, and some have not, some have been "tribal" and some have not.
But communism and other "non-religious" ideologies happen to share most of the underlying assumptions of "religion": that they have all the answers, for all people, for all time, everywhere, and that everything will be fine once their "truth" is universally acknowledged. This kind of thinking leads to war and intolerance more often than not.
But you can't just look at total body counts when talking about the 20th century: there were a lot more people alive then - you have to look at "per capita slaughter" if you want to claim to be objective. What distinguishes the larger body count of 20th century ideologies compared to, say, the wars of religion in Europe (one of which, for example, the 30 Years War, killed 1/3 the population of Germany), is that the 20th century had larger populations of "people in the way" and better technological means at the disposal of warring parties to more efficiently wipe them out.
That's "progress" for you.