By definition slaves are ill treated because slavery in itself is terrible. To be clear here, even if your master was individually kind you existed in a system where you had no rights, if you were a witness to a crime you would have to be tortured during your testimony to prove it's true, where you were not fully protected from the law by assaults from any free person, and where you could be sold at any point. It was considered perfectly acceptable for a "kind" master to rape and beat you at any time. Slavery is always a horrendous thing.