If the Israelites didn't already know that murder and robbery were wrong, they never would have survived long enough as a people to "receive" the 10 commandments in the first place. I mean, think about it. Why was Moses mad at Aaron for building the golden calf while Moses was up on the mountain with God? It's not Aaron's fault that he had no commandments and therefore no morality. The same goes for Cain. He had no commandments, therefore he must have thought that killing his brother was perfectly acceptable behavior.
Except, of course, that's bull****.
Why is having a code of morality justified whether God is placed at the head of it or not? Because just as species evolve, so does society. It is the exact same mechanism, one of trying rules, discarding some, adopting others... changing as conditions change.
Morality helps society function. It provides order. It regulates acceptable behavior. It settles disputes. The better it does those things, the more the society flourishes. The more a society flourishes, the more it's morality spreads and therefore the more prevalent the people who believe in it become. A physically inferior group of humans working together, under a code of self sacrifice and discipline, will invent a better weapon, produce a better crop, build a better defensive wall, train a superior army... and will therefore always crush a physically or even mentally superior but less organized foe. That is the power of society and the morality that helps drive it, as history has shown over and over again. "God" is merely the figure at the top of the morality pyramid, a supernatural policeman who inspires the fear and awe that best incentivises the following of the "rules."
Certain basic rules (such as the moral "wrongness" of murder and theft and the moral "rightness" of self sacrifice) make so much common sense they have evolved in virtually every society on Earth, just as, say, two eyes make evolutionary common sense. The violation of those rules is "wrong" the way a mother pig refusing to nurse her young would be "wrong," in that it stands in violation of the evolutionary principles that allowed pigs to flourish. Humans have only added an extra layer of complexity and understanding to their behavior, which gives us the strength to adapt quickly but also the "burden" having enough intelligence to question and doubt.
This kind of understanding of morality might not appear to be as "absolute" as the God of the Bible, but that's only if you don't know that the God of the Bible is as shifty and ever changing as society itself (and for good reason. Only a relevant God can perpetuate the society that believes in him). What's particularly interesting about Christianity is that it has held on to the Old Testament. This has value, I suppose, in that it allows itself to be painted as the fulfillment of prophesy. But to accept the whole Bible is to accept that God has changed his mind about many things that were previously important. The ridiculous detail and punitive nature of Numbers and Leviticus, after all, is strikingly different from the broad-stroked hippie philosophy of the New Testament, in which one need only accept Jesus Christ as one's personal savior (if you are of one denomination), or to follow on from this acceptance not with a strict adherence to a minutely detailed code of conduct, but by the repentance of sin (which either way has been taken care of) and the doing of good works that shows your commitment to the faith. The way the "Golden Rule" is interpreted today stands in stark contrast to the ritual slaughter of whole peoples and of the rape and slavery condoned by God or -- in the case of the plagues of Egypt -- the outright murder of children by the Holy Spirit.
All of the major Western religions are outdated relative to a modern context, and it's no surprise to me that the largest growing "religion" in the United States is athetism (though it's ridiculous to consider the negation of religion to be religion). Poll after poll shows that strict Judiaism has largely been abandoned in favor of a more secular approach. Christianity is fighting a losing battle by directly challenging scientific discovery and standing it's ground on a few arcane passages. Islam has been bisected into a pre-Middle Ages barbarism and a more liberted -- though still somewhat severe -- modern faction.
It probably won't be in any of our lifetimes but it seems to me a new "God" is coming, and that sometime in the future the Bible will be treated how we treat The Illiad and The Odyssey today. Eventually something will be invented to fill the void but not be saddled with the shabby hocus pocus of the Bible.
It will still -- just as the Bible is -- be an invention of man. It will still serve the same purpose of societal control, of divine inspiration for it's rules. It will still be possible to see the system for what it is and reach a more complete awareness of the mechanism.