Without an absolute evil, there cannot be an absolute good. Since Christianity as a religion propagates that Jesus is ultimate good, there must be an ultimate evil to counteract that.
This is not necessarily a bad thing, mind you. It's just the way it is.
yes, satanism and evil was created by Christians to drive fear into those of non believers to boast their "superiority" much like they persecuted pagans and murdered them for not converting (IE. Salem witch trials)