The theme of Satan is a very old concept tracing back throughout history and predating modern religions. It is a very powerful part of human nature, the idea of evil or darkness. We aren't talking about a horned guy in a hot place, as most religions make it clear that the devil can take many shapes.
As for Wals's point, I like Orogun's observation about Satan being a primarlily New Testament character to separate evil from God. From a theological standpoint it makes sense, as proselytizing someone with a vengeful God is going to be much harder than a compassionate one.
It is odd then, that Satan receives no compassion, but perhaps his very existence is a mercy given by God.