Chr*stians think he's a "fallen" angel who "rebelled" against G-d and mucked everything up. They even call him "the 'gxd' of this world." He is not a fallen angel, he is not in rebellion, and he never was "gxd" of anything. G-d is in control of everything, including evil. ***********
I'm not sure how you came to this understanding, but it is not correct. We Catholics have never considered Satan to be "a" god or God.