This is the question I always ask Christians and I never get a response. Why the fall?
Assume an omni-max deity. Not a popular assumption around here, but this conversation takes place with a Christian and they assume an omni-max deity.
(Aside - omni-max means "omnipotent, omniscient, omnitemporal, omni-whatever")
So this perfect omni-max deity would create the universe. It would, of course, do a perfect job. That means that everything was created so that it had a place and function that was ideal for it.
Then why would anything created deviate from that role? A cat likes to chase mice, a cat doesn't like to swim in the ocean, and you don't need to tell the cat "don't swim in the ocean, you'll drown, go chase mice instead."
Usually when confronted with this problem re Adam and Eve, the Christian will drop back ten and punt by saying "well the devil..." but that makes the problem worse. The Devil, supposedly the greatest of the angels and the one closest do God, would be even more so properly in place and wanting to do good.
"But free will..."
No. I am able to choose to drink Draino. I choose not to because I know the consequences if I do so. For the Devil to rebel is somewhat akin to me choosing to drink Draino.
"Oh, but if he doesn't want to rebel that means he doesn't have free will..."
Ok, Christian, if you don't drink Draino it proves you aren't able to choose to do so. Go prove to me you have free will.
I always picture the situation with the Devil going something like this.
Sataniel (his pre-fall name) is in heaven, miserable, crying his eyes out. Michael comes up and says "what's wrong, friend Sataniel?" "Oh, God gave me free will, so now I have to rebel even though I don't want to." "But if you don't want to doesn't that mean that you won't do it?" "Oh no, human apologists said if I don't do it, even though I don't want to, it proves that God didn't give me free will. I have to do it to prove I can do it even though I don't want to." "I'm confused, friend Sataniel." "So am I friend Michael. Oh well, even though I don't want to I guess I better get going on the rebellion."