Sometimes I wonder, If God really and truly wants to defeat Satan, which almost everyone agrees that he does, why doesn't God just snap his fingers and make Satan not exist? It'd be the simplest freaking thing to do. And no more Satan means no more corrupted humans. Methinks God's in need of the Staples Button.
I personally believe that Satan and Hell don’t exist in the tradition sense. I believe that Hell is on Earth and that Satan is the evil every person has the potential to carry out. Hence, God can’t get rid of Satan because he would be getting rid of us. Just a thought.
(Provided that they do exist), they cannot exist without eachother. It's a dichotomy; good and evil, yin and yang, god and satan, a glass of cool milk and a red hot kitchen. They're polar opposites, however, they cannot exist without the other. You cannot know what is good without knowing what is evil, and vice versa. Also, humans always have been corrupt, and always will be.
He needs Satan around to scare humans, To keep himself in business.
The devil is the church's friend, He has helped keep it in business all these years.