I doubt there is a clear answer to that question. Slavery was practiced worldwide, and though much of it has gone underground it still is to one extent or another. The treatment slaves receive has always varied widely with the specific case at hand. While there were (and where it remains openly instituted are) laws and customs governing it, the behavior of the slaveowners depended more on personal ethics than on those rules and so varied over a broad range, just as does that of employers today. Some were benign, some stern, some cruel, some sadistic. The culture of the civilizations or countries of which you speak was a factor, too, and so depending on whether it valued human life highly or not so much would have affected the treatment of slaves.
Economics, however, was more important than any of those. Slavery, though it did not always go under that name, existed outright in societies which needed it to produce their goods and services. European serfdom, for example, was a form of it. When industrialism began to replace agrarianism as the dominant economic system in the western world, slavery was outlawed in one jurisdiction after another not primarily out of the moral considerations used to justify it but in order to create a competitive labor market. Conditions for the workers did not necessarily improve from those of slavery, but it was a step toward modern economics.