“Son of Adam,” said Aslan, “you have sown well. And you, Narnians, let it be your first care to guard this Tree, for it is your Shield. The Witch of whom I told you has fled far away into the North of the world; she will live on there, growing stronger in dark Magic. But while that tree flourishes she will never come down into Narnia. She dare not come within a hundred miles of the Tree, for its smell, which is joy and life and health to you, is death and horror and despair to her.”
. . . Aslan suddenly swung round his head . . . and fixed his large eyes on the children. “What is it, children?” he said, for he caught them in the very act of whispering and nudging one another.
“Oh—Aslan, sir,” said Digory, turning red, “I forgot to tell you. The Witch has already eaten one of those apples, one of the same kind that Tree grew from.” He hadn’t really said all he was thinking, but Polly at once said it for him. (Digory was always much more afraid than she of looking a fool.)
“So we thought, Aslan,” she said, “that there must be some mistake, and she can’t really mind the smell of those apples.”
“Why do you think that, Daughter of Eve?” asked the Lion.
“Well, she ate one.”
“Child,” he replied, “that is why all the rest are now a horror to her. That is what happens to those who pluck and eat fruits at the wrong time and in the wrong way. The fruit is good, but they loathe it ever after.”
“Oh, I see,” said Polly. “And I suppose because she took it in the wrong way it won’t work for her. I mean it won’t make her always young and all that?”
“Alas,” said Aslan, shaking his head. “It will. Things always work according to their nature. She has won her heart’s desire; she has unwearying strength and endless days like a goddess. But length of days with an evil heart is only length of misery and already she begins to know it. All get what they want; they do not always like it.”
I think there are a whole lot of applications you can make from this one little passage, but I’ll leave it to you to figure out what those are.