AMUSING, INTERESTING, OUTRAGEOUS, or PROFOUND

Look, I'm no Christian, but I can point to the obvious answer to this:

Slavery is part of the free will of humans. It's humans doing things to other humans, in which God will, in general, not intervene. It all stems from the original sin and humanity as a whole living with the consequences of its (as a whole) actions. It's such an easy, toss-away answer to give for the absolute worst of human evil and a supposed loving God's complacency, but it's there, and it's easy.

I prefer Stephen Fry's example for condemnation:

"Yes, the world is very splendid but it also has in it insects whose whole lifecycle is to burrow into the eyes of children and make them blind. They eat outwards from the eyes. Why? Why did you do that to us? You could easily have made a creation in which that didn’t exist. It is simply not acceptable."

Also:

"I would say: ‘bone cancer in children? What’s that about?’

“Because the God who created this universe, if it was created by God, is quite clearly a maniac, utter maniac. Totally selfish. We have to spend our life on our knees thanking him?! What kind of god would do that?”

That's not a matter of free will. That's a matter of creating a world that is hostile to those who are innocent, even without the iniquity of Man.