The religion scholar, Elaine Pagels has written a book on this topic:
(This started as a comment to Gomedome's recent post, then grew.) The Origin of Satan. I recommend it to anyone seriously interested in putting Satan in perspective. As for giving credit, responsibility and/or blame for the concept of and Evil Counterpart to God, I glare at the Folks who followed Mithra, who really gave a push to the notion of divine duality, which has permeated much of religion since long before the sprouting of the Judeo-Christian belief-tree. Not that older religions didn't... Sign in to see full entry.