I don't see the following, but anything ethical based would usually work
I.e.: They are mistreated, one human's power over another is wrong, our nation's image is tarnished.
Avoid arguments about profit since they would argue in favor of slaves, and be careful about religion since it swings both ways (ex Slavery is a sin vs slavery allows slaves to learn religion) Hope this helps
If this doesn't help please comment the possible answers so I can further assist
