Here is another excerpt from Mere Christianity, in which C. S. Lewis speaks of morality and our understanding of good and bad:
When a man is getting better he understands more and more clearly the evil that is still left in him. When a man is getting worse he understands his own badness less and less. A moderately bad man knows he is not very good: a thoroughly bad man thinks he is all right. This is common sense, really. You understand sleep when you are awake, not while you are sleeping. You can see mistakes in arithmetic when your mind is working properly: while you are making them you cannot see them. You can understand the nature of drunkenness when you are sober, not when you are drunk. Good people know about both good and evil: bad people do not know about either.