Why? How on Earth can evil ever be good?

An Evil Thought Experiment: A world exists called Goodyworld, a place where evil does not exist. Goodyworld is inhabited by the Goodians.

The Goodians, then, do not have free will. For free will implies considering all possibilities, even evil ones. And evil is not permitted on Goodyworld.

Therefore, if one wishes to have free will, one must accept evil as, literally, a necessary evil.