It is simply good /spoiled.

On evil I think () one may be quite rid of the old haunting suspicion—which raises its head in every temptation—that there is something else than God—some other country . . . into which He forbids us to trespass—some kind of delight //which He “doesn’t appreciate” or just chooses to forbid, but which would be real delight /if only we were allowed to get it. The thing just isn’t there.  [Whatever we desire] is either what God is trying to give us /as quickly as He can,or else a false picture of what He is trying to give us—a false picture //which would not attract us /for a moment  /if we saw the real thing.  Therefore God does really in a sense contain evil—i.e., contains [what is the real motive power] /behind all our evil desires.  He knows [what we want], even in our vilest acts: He is longing to give it to us.  He is not looking on /from the outside /at some new “taste” or “separate desire of our own.”  Only because He has laid up real goods /for us to desire are we able to go wrong by snatching at them in greedy, misdirected ways.  The truth is that evil is not a real thing at all, like God.  It is simply good /spoiled.  That is why I say () there can be good without evil, but no evil without good. You know [what the biologists mean by a parasite—an animal //that lives on another animal. Evil is a parasite.It is there only because good is there /for it to spoil and confuse. Thus you may well feel that God understands our temptations—understands them /a great deal more /than we do.  But don’t forget Macdonald again—“Only God understands evil and hates it.”  Only the dog’s master knows [how useless it is to try to get on with the lead /knotted round the lamp-post.  This is why we must be prepared to find God implacably and immovably forbidding [what may seem /to us very small and trivial things].  But He knows [whether they are really small and trivial].  How small [some of the things //that doctors forbid] would seem /to an ignoramus. From The Collected Letters of C.S. Lewis, Volume II Compiled in Words to Live By