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.
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.
Can you explain how this is objectively true? How would we reason a Martian, a Roman, or an Aztec into believing this is morally correct?
We believe it because it is absolutely true in our moral system, which is not some grand system arrived at by reason alone, but by changing social norms and mores - fashions, basically.
To some degree, 'morality' is based in genetics, human being social animals - we all mostly start out with some broad in-group altruism. But the expression given to this varies wildly throughout human societies - people have believed it was perfectly moral, good, and correct to sacrifice slaves to the sun god, and other people have believed that all humans are equal. Neither one of these are objectively correct in any meaningful sense, although I much prefer living in the latter system. But of course, I would say that, being inundated from birth.
> The amount of people who are unjustly prevented from living their life is an absolute number, as a percentage of the population.
The idea that people can be unjustly prevented from living their life is, itself, a moral judgement.