As humans we are tempted by Satan every day. Sometimes we make God happy. And other times we make Satan happy. The devil is a good way to rationalize unloving behavior and protect our ego. It's easier to blame Satan for tempting or deceiving us rather than taking all the responsibility ourselves.
It's most likely that God is all there is. If he is everywhere, then certainly he wouldn't want to share the same world with his adversary. Instead of being an angel, Satan is likely a choice we make to live our life without God's love and light.