Do you think that Satan would not cease to exist the second God wills it?

Therefor, Satan is instrumental to God's plan. That doesn't mean Satan is good, but that God's plan seems to require him for His plan for reasons He has not spelled out to us.

One can deduce that, God raising a truly concious and seperate Elect people to be Christ like after the end of this world, that a temporary existence of evil and Sin is necessary to that end.

Therefor, God authored evil into Creation, and is the author of evil.