Sorry for the late reply. I'll just address the fact that the slave being sacrificed to the sun gods was a slave in the first place, because it's well known that totally messed up things are justified by religion, and that's a whole different thing. The fact that this sun god society has slaves in the first place is sufficient for me to make my point.
Being a slave means that your masters consider your life to be less valuable than theirs. So even though you're a human being, they deny you rights and treat you like a lower form of life. This is what I meant by unjust suffering, perhaps I phrased it wrong, but my point was that it's simple math. Is this life form a human being? Are they well intentioned? Do you prevent them from leaving or harm them regardless of their good intentions? Does your society condone your actions? If so, morally speaking, you're living in an objectively worse society. You're doing harm to someone of your own species who doesn't wish harm on you.
> Being a slave means that your masters consider your life to be less valuable than theirs. So even though you're a human being, they deny you rights and treat you like a lower form of life.
The same can be said in our society about criminals or the poor. Not universally, but slaves were not universally seen as "lower forms of life" either.
> Is this life form a human being? Are they well intentioned?
The use of "well intentioned" here is a catch-all for being able to say "anyone who does not follow my moral framework may be subjected to the punishments of my moral framework." It is equally capable of condemning the well-intentioned thief stealing food (or money for food) to live, with no ill will but no belief in other opportunities for getting by, and also the well-intentioned parents who refuse medical treatment to a dying child due to the belief in their religion that God will heal the child without the corrupt manipulations of the material world. There is no objectivity here.
Being a slave means that your masters consider your life to be less valuable than theirs. So even though you're a human being, they deny you rights and treat you like a lower form of life. This is what I meant by unjust suffering, perhaps I phrased it wrong, but my point was that it's simple math. Is this life form a human being? Are they well intentioned? Do you prevent them from leaving or harm them regardless of their good intentions? Does your society condone your actions? If so, morally speaking, you're living in an objectively worse society. You're doing harm to someone of your own species who doesn't wish harm on you.