> There are no cases where slavery is good, regardless of who does it or who it's done to. The same goes for genocide, murder, robbery and so on.

Have you heard of prisons? We use a different word for it so as to not introduce the (obviously false) semantic "contradiction".

Perhaps if you want to say that forced labor is bad even after conviction, then let's say "kidnapping is bad when done by bad people, kidnapping is good when done by good people [to bad people]". Ta-da, you've invented prison.