The Magician's Nephew

How could evil originate in a good world? Or did it? In The Magician's Nephew, Lewis solves that theological conundrum by making the source of evil a malevolent invader from another world. Lewis has a comparable device in Perelandra. 

I remember a Bible scholar who said the Tempter in Gen 3 performs the same function. Since Eden was initially devoid of evil, it had to enter the garden. The source of evil lay outside the garden rather than the inside the garden.

Although that may finesse the proximate source of evil, it only pushes the question back a step. It can't explain the ultimate source of evil. How did the malevolent invader become evil in the first place? How did evil originate wherever he came from? 

The issue is sometimes framed in terms of how a holy or perfect agent could ever find evil appealing in the first place. 

It's like asking how a movie villain became a villain. At one level, there may be an explanation inside the plot or narrative. There may be a backstory about some pivotal event that took him in the wrong direction. 

At another level, outside the story, he's a villain because the director had the idea of a villainous character, and he turned his idea into a movie. It began in his mind. The villain was originally a thought. The villain in the story objectifies the director's imagination. At that level, he does dastardly things in the movie because he does dastardly things in the director's imagination, and the movie character is a projection of the director's imagination. 

There's the additional fact that while Adam was initially sinless, that doesn't mean there was no room for improvement. A quest for knowledge isn't inherently wrong. Intellectual curiosity is a good thing. 

In this case, it's forbidden knowledge, but that combines something innocuous with something prohibited. There can be wrong ways to acquire something good.

Moreover, certain kinds of knowledge are corrupting. Getting inside the mind of a serial killer is corrupting. Likewise, learning about evil by doing evil is corrupting. Then there's second-order evils where an agent must commit one kind of evil to be in a position to experience another kind of evil. Some kinds of knowledge are safe for God but dangerous for creatures.