Steven Pinker wrote quite the tome on this subject. Its worth the read. "I argue that despite impressions, the long-term trend, though certainly halting and incomplete, is that violence of all kinds is decreasing. This calls for a rehabilitation of a concept of modernity and progress, and for a sense of gratitude for the institutions of civilization and enlightenment that have made it possible." http://edge.org/conversation/mc2011-history-violence-pinker