Why God Did Not Destroy Satan

God could only use methods that were consistent with truth and righteousness. Satan could use what God could not—flattery and deceit. So it was necessary to demonstrate before the inhabitants of heaven and all the worlds that God’s government is just, His law perfect. Satan had made it seem that he himself was seeking to promote the good of the universe. His true character must be understood by everyone. He must have time to reveal his true self by his wicked works.BOE 10.5

He declared all evil to be the result of the divine administration and it was his own purpose to improve God’s law. So God permitted him to demonstrate the nature of his claims, to show the working out of his proposed changes in the divine law. His own work must condemn him, and the whole universe must see the deceiver unmasked.BOE 11.1