"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 starting point that you ask for I think would be the immediate family in whom our genes are largely invested. We likely evolved to not want our immediate family members to die violently because those genes could easily be sustained. From this point we can extend it our to our non-immediate family members. From this point we can further extend it out to friends/trading partners and so forth, ever expanding our moral circle. Since we were invested in all these people either genetically, emotionally, or economically we may have rationalized reasons why it was "WRONG" to kill them, and hence the foundation of morality is formed. Even though it pains me to consider that the foundation of morality is just a rationalization, this is the conclusion I am forced to accept given my assumptions.
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]