"You can be good for the mere sake of goodness: you cannot be bad for the mere sake of badness.  You can do a kind action when you are not feeling kind and when it gives you no pleasure, simply because kindness is right; but no one ever did a cruel action simply because cruelty is wrong--only because cruelty was pleasant or useful to him.  In other words badness cannot succeed even in being bad in the same way in which goodness is good.  Goodness is, so to speak, itself: badness is only spoiled goodness." Mere Christianity

Lewis's point is that there are not two equal and opposite powers in the universe: good and evil.  If there were, then why do we call one good and the other evil?  Is it not evident that we are judging the two by some higher standard?  And that higher standard is God?

So then, good is original; evil is only a false copy of the good.  Evil feeds off of goodness; it takes goodness and twists it into an unnatural shape.

Take the goodness of human sexual pleasure as an example.  Evil takes that pleasure and perverts it--uses it--at the wrong time, in the wrong place, with the wrong person, from the wrong motive.  In fact, the pleasure still left in the sexual act, when so perverted by evil, is the only good thing about it, because pleasure itself is a good.

So what will happen when evil can no longer feed off the good?  What will happen when good and evil are completely separated?  Evil will starve.  Having no life-blood on which to survive, evil itself will die.

This separation the Bible calls the final judgement, and the place where evil dies--hell.  Heaven will be the place where evil can no longer corrupt the good.

"And the devil, who deceived them, was thrown into the lake of burning sulfur . . . Then death and Hades were thrown into the lake of fire. . . . Then I saw a new heaven and a new earth . . ." Revelation 20:10-21:1

"May He support us all the day long, till the shadows lengthen, and the evening comes, and the busy world is hushed, and the fever of life over, and our work is done.  Then in His mercy, may He give us a safe lodging, and a holy rest, and peace at the last."  John Henry Newman 1801-1890