As I've pointed out on this very thread, rules for living together are so important people throughout the world came up with comparable systems without Divine intervention. Attributing such rules to the Almighty might make them more palatable to those unable or unwilling to think rationally (which, unfortunately, is the great majority of the species), but that does not necessarily make them Divinely inspired.
I appreciate your skepticism. Obviously you are not a believer and obviously you make no claim to be a follower of Jesus. I have no problem with atheists and agnostics who profess a disbelief in the authority of Scripture. My problem is with those who claim to be followers of Jesus, yet they do not believe his words.
I have a problem with those who profess to be Christians, yet they publicly profess to disbelieve the Word of God and call God a liar.
"You forget we are social critters. We live in groups because we need cooperation among individuals to survive"
"They are out-competed very effectively by societies that promote co-operation"
You are still dealing in the area of self interest, not altruism or common good. The Carnegies, Rockefellers, Fords, Soros, Gates, Stalins, Pol Pots, China, North Korea, Iran, Saudi Arabia all use cooperation to suit their selfish ends but when necessary they, at heart, are ruthless self monopolists right out of the Ayn Rand school of the strong individualist survives because there is nothing greater than themselves or a transcendant being they must one day answer to.
How can you trust someone whose ethic is what he thinks is right at the time but might change in the next minute do to a changing circumstance? Who is to say that the looting actions in New Orleans during and after Katrina are not justfied by the ethic of the have-nots getting their "just" do? Certainly in those circumstances cooperation or "the Golden Rule" was suspended in the face of self interest.