I disagree with your comments regarding the irrelevance of morality in human behaviour.
Social Darwinism has been used to justify the most base behaviour by everybody from the eugenicists of the 1940s to the free-market libertarians of today, but the actual basis for SD is not logical. You espouse the arguements made by the religious in the face of rising atheism; if there's no god, then nothing matters, so we may as well behave as savages. To be an atheist is to be a savage. As Dawkins is fond of pointing out when confronted by this line of reason, are you really suggesting that the only reason religious people refrain from murder and rape is because they're scared of God? I take it that you are yourself an atheist (as am I) and are viewing this element of human behaviour through the prism of your disenchantment with the moralizing hypocrisy of the church, but you are mistaken. If anything, secularism has been an overwhelmingly positive force for tolerance and freedom; you cannot argue that human behaviour is becoming less civilised - how many vegetarians do you think there were in England 100 years ago?
There is such a thing as morals. Although they're intangible, they are none the less universal. The same moral standards apply across all human societies regardless of level of socioeconomic or technological development, race, or religion. I'm not going to go into the detail, but these things have been objectively studied by psychologists and philosophers, and they find that when presented with moral scenarios (is it morally justifiable to kill an innocent person in order to save the lives of 5 others? etc) that the answers are the same. You may argue they're not based on anything real, because there is no god, but your own arguement (we're just doing what comes naturally) is based on the same premise; that intrinsic human behaviour is a real force. As you say, justice exists because it has been invented.
More importantly, even if it is in human nature to kill and conquer, that does not mean we have to shrug our shoulders in the face of the inevitable. It is in human nature to enslave the vanquished, to rape, to steal, but we have power over our impulses. You are absolutely correct that a certain section of society will follow these impulses, and will not regret them. The murderers of Darfur probably will die old and powerful, but I don't think that we should sell them weapons just because they're always going to be out there.
In the 21st century we do have alternatives to hitting civilians with pointy pieces of metal when we're not happy with the way their leaders behave, and our economy really does not hinge on selling exquisitely designed killing machines to second rate despots obsessed with keeping up with the Joneses.