Mere Christianity, in The Complete C. S. Lewis Signature Classics

pp. 32-33

Lewis explains the Augustinian claim that goodness is original, but badness only spoiled goodness.  “In reality we have no experience of anyone liking badness just because it is bad.  The nearest we get to it is in cruelty.    But in real life people are cruel for one of two reasons—either because they are sadists, that is because they have a sexual perversion which makes cruelty a cause of sensual pleasure to them, or else for the sake of something they are going to get out of it—money, or power, or safety.  But pleasure, money, power, and safety are all, so far as they go, good things.  The badness consists in pursuing them by the wrong method, or in the wrong way, or too much.  I do not mean, of course, that the people who do this are not desperately wicked.  I do mean that wickedness, when you examine it, turns out to be the pursuit of goodness in the wrong way.  You can be good for the mere sake of goodness: you cannot be bad for the mere sake of badness. . . . Goodness is, so to speak, itself: badness is only spoiled goodness. . . . Evil is a parasite, not an original thing.”