Well said, but if player's make in-game choices to be involved in slavery there better be an incredible story-arc that ends somewhat morally positive (IRL terms). It's still a fantasy game after all.
I played in a Dark Sun 4ed campaign over 10 years ago and we had to end it. It was due to slavery and human trafficking. We the PCs found ourselves involved in the slave trade and didn't have a reasonable in-game way of getting out of it. It was a trap the DM admitted to and the decision to...