Friday Quote from Mere Christianity by C.S. Lewis:
If Dualism is true, then the bad Power must be a being who likes badness for its own sake. But in reality we have no experience of anyone liking badness just because it is bad. The nearest we can 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, as 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 people who do this are not desperately wicked. I do mean that wickedness, when you examine it, turns out to be the pursuit of some good in the wrong way.
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 of 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. And there must be something good first before it can be spoiled. We called sadism a sexual perversion; but you must first have the idea of a normal sexuality before you can talk of its being perverted; and you can see which is the perversion, because you can explain the perverted from the normal, and cannot explain the normal from the perverted…
And do you now begin to see why Christianity has always said that the devil is a fallen angel? That is not a mere story for the children. It is a real recognition of the fact that evil is a parasite, not the original thing. (emphasis mine)