I don't believe in satan. The idea as expressed in the various posts here seems to be a construct for some long running cosmic battle.
What's interesting is why humankind through various religions has thought to invent such an entity - is it to justify certain behaviours ("I couldn't help it, Satan made me do it!/I did it in God's name!") or just because in every good story we need a baddy as a foil for the good guy.
Sal