That is the modern view.
Historically, slavery was a response to agricultural surplus. Previously, war-captives were executed; the victorious tribe could not afford to feed them and didn't want to turn them loose to seek their revenge. They were often sacrificed to the tribal gods.
With food surpluses, tribes realized they could get useful work out of their captives, instead of simply killing them. Slavery was thus, originally, a more humane alternative to execution of war-captives, once it became economically viable.
Some slaves eventually were allowed to join the tribe as free men, after a period of time and the establishment of trust. Others might have remained slave the rest of their lives. I expect most of them found it preferable to being put to death in some ritualistic (and often painful or horrifying) manner.
Native American Indians practiced this form of slavery, btw.
IN that sense, slavery was an improvement over what had come before, in historical context.
As I mentioned in another post, in ancient times there were many eras/cultures where almost everyone was either a servant or a master. That is, you were either a man of property, or you were a servant to a man of property. IN many cases, a servant was required to fulfill a set number of years as a servant, and then decide whether he wanted to go free or remain in the service of his master. In modern times many wish to equate this practice with slavery, but it isn't comparable really.
Historical context is important.
