Partly because we can see spectacular violence (9/11, for example) on TV, we tend to overestimate the violence of our times.
When Rome conquered Gaul, roughly one-sixth to one-third of the population was killed, about the same number enslaved, and the rest spared. And this sort of thing was the norm rather than the exception. Today that would be like 50-100 million people in the US being killed, another 50-100 million enslaved, and the rest spared. Despite WW1 and WW2, the 20th century really was the least violent in human history, and weird as it seems, we really are headed towards world peace!
Steven Pinker has more on this here [http://www.edge.org/3rd_culture/pinker07/pinker07_index.html].