Truly this confusion compounds itself. Satan is the whisperer, the reject, the liberal, powerless except for knowledge and for humanity. Strength, darkness, silence, cacophony, authority, hell, heaven all come from the Lord, for all creation comes from him and evil is a privation thereof. I see my favourite Euronymous quote in this thread, yet the Christianity that he despised is modern Christianity which is a front for liberalism (as far as people are concerned). Ironically his position was closer to genuine faith.
What is Satan? More importantly what is he not? Not the dark half that is for sure, but that is what he is often considered. The rebellion of Satan is one of petulance and ego, not will, for will is a godly thing. The horned Lord is not Satan, though I am sure he would love to be considered as such!