Morality is a human construct (actually, I think that was the title of a GD thread a million years ago). In nature, whatever you can get away with that supports your own procreation in the short term, and your species’ procreation in the long term, is fair to both victor and vanquished.
So, if you consider humans to be animals and just another child of the universe, then morality is simply a crutch for the weak, and the strong should do as they wish according to their own interests. In that system, slavery is never wrong as long as it serves the short and long term aims of the slaver. This thought process is alive and well among the human population throughout the world.
Self-preservation requires concessions from the strong, however. I think that’s where morality comes from–everyone agrees to certain constraints on behavior to keep the weak from growing desperate and overpowering the strong, and to keep the strong from expending too much energy on keeping themselves strong. Everyone digs a little bit of peace. The concessions required of the strong change as society and its collective sensibility changes–morality changes. It is important to embrace this, I think, because otherwise history becomes a seething mass of rampant immorality and anarchy if you try and understand it with modern sensibilities. And history MUST be understood given it’s utility as a scry into the future.