Some claims that "satan" simply means "adversary" - the "nay-sayer" or the "voice of opposition" - rather than a specific person... and certainly not the embodiment of evil.
E.g. God plans to do something, and all the minor gods and angels agrees... except one, who voices his opposition against the plan. Then he is "satan" - God's adversary, the one who disagreed with God... this time. Another time, it may be a different one of God's advisers that voices opposition and becomes "satan".
There are some stories from the Bible (like the story about Job) which make more sense with this view, that "satan" just refers to whomever opposes/disagree/questions God at the moment, rather than one particular entity. It certainly explains why God seems to repeatedly allow "satan" into His midst, and use him as an adviser along with other advisers.