Something I dont get about some Satanists

That you believe in Satanism, you believe in satan aka the devil, and yet you dont believe in God…how is this possible? I mean, you dont have to side with God, you can side with the devil, thats cool, thats your choice, but to say he doesnt exist, and yet satan does, that just dumbfounds me. I mean satan after all, is a fallen angel…an angel from God, so to believe in one, you must believe in the other…that is only logical and makes sense, without one, there isnt another…..please explain to me, your logic in this?