From a sociological perspective, I have always been partial to the theory that Satan as the adversary of God arose when the Jews came into direct contact with Zoroastrianism during the Babylonian exile. In that religion the God of Good (Ahura Mazda) is directly balanced by a god of evil (Ahriman). Some Jews, in an effort to synthesize the religions--and in an era when tightly controlled doctrinal arbiters were rare--took the angel Satan (who is reasonably clearly not the banished enemy of God in the Old Testament, and is not in the Jewish faith generally) and elevated him.
They also decided to limit his power, in a way that makes little sense if God is good, since God could crush Satan and his evil without effort instantaneously if He wished...meaning that he must want Satan and his evil in the world.
It is possible, if one believes, that Satan, rebel or not, is still largely following God's original plan for him, by testing the faith of men. Perhaps God is too soft-hearted, and decided that He needed a free-willed and independent Ha-satan to do the job right, and given free will Satan turned to evil himself.
Personally, even in my most Christian moments, I don't believe in Satan as the enemy of God.