Though Screwtape is a work of fiction, Lewis gives us a "true picture" of Evil, its motivations, "values," and methods. It is a simply chilling book.
Lewis said (probably elsewhere, but I'd have to track down the source) that Satan is not the opposite or foil of God. The reason is God is uncreated, eternal; but Satan is, like man, a creature of God -- an exalted archangel who rebelled, and took certain ranks of angels into rebellion with him (except for Abdiel, one of the angels in Lucifer's regular ranks, who remained faithful to the Lord). So Satan's opposite or foil can only be another exalted archangel (i.e., it must be another creature) -- the Archangel Michael, for instance.
God and Satan are not of equal ontological rank, and therefore cannot be said to be equal opponents or "opposite" in the manner that we say hope is the "opposite" of evil. As two equally active ontological principles, hope and evil can be said to be opposed to each other.