"Lowly Undergrad, early societies didn't have this idea of reducing violent death to zero - through what mechanism did they acquire this belief, given that they didn't start out with the idea that it was "moral progress"?"
While it is certainly difficult to imagine the mindset of people who existed ten of thousands of years before us, I think since they were still human beings, we can assume they were somewhat similar to you and I. From this basic assumption think we can look toward Peter Singer's philosophy of the moral circle. The s...
I don't think anyone can really argue that a large-scale decrease in global violence and violent death is a sign of moral progress. So I must point to this Steven Pinker conference where he lays out some statistics showing the gradual decline of violence and violent death throughout our history: http://www.ted.com/index.php/talks/steven_pinker_on_the_myth_of_violence.html
Can I request a little more explanation? Why should I adopt this method? [/sarcasm]