Imagine A World Without Religion.

[Tempus]

We had a microcosm of what a world would be without religion in the 20th century.

Weren't those regimes more enforced atheism in addition to a world without religion? A world without religion could be all atheistic, atheistic and theistic*, or even totally theistic. I certainly wouldn't look at a period of history (or even countries today) where religion was forced onto people and say that is the world with religion. Also, while not accounting for the entire population, levels of irreligion in countries like Japan and Sweden are quite high and they're not despotic hell holes.

*or deistic, pantheistic, etc.

Half the violent deaths in history arose from that, but then again 75% of violent deaths happened in that century as well.

I'm not sure where you're getting these figures (though I'm not disputing them). Do they take into account population growth? Steven Pinker, who argues that violence is actually in decline, says the following:

• "If the wars of the twentieth century had killed the same proportion of the population that die in the wars of a typical tribal society, there would have been two billion deaths, not 100 million." - A History of Violence