* Treating women and men as equals is in the best interests of men.
* Slave-holders are harmed by the ownership of slaves.
* Race-based oppression damages the oppressor.
And so forth. I don't think that the institution of slavery is only bad for the people enslaved or even "does enough damage to certain classes that on average humanity is better off without it". I mean it is bad for absolutely everyone including the people who appear to gain from it. This is counterintuitive, because it looks like a good deal for slave-owners, who get the benefit of the slave's labor at "no cost". Of course, "no cost" is wrong: there is the cost of feeding, clothing, and sheltering the slave (dead slaves do not produce labor). And the cost of insuring the slave remains at work, whether this is by imprisonment, guards, social conditioning, societal reinforcement, or whatever: it is not free, even if the costs are hidden to the individual slaveowner. The slave-owner will realize some short-term gain from using slaves. But if one set up two societies, identical except that one was free and one had slaves, I think that the free society would do better across every class of people in the long run; in the very long run, the poorest 10% in a free society would be weatlhier than the richest 10% in a slave-holding one.
I'm not sure this is demonstrably true in any unambiguous way. I'm pretty sure that you can make a case that some moral evils were the "best practice for success" in a given era. But I still have this feeling like the course of human history is lurching unevenly towards a better world, one with more freedom for all not just because freedom is a good thing (which it is!) but also because that's what works. That the tinpot dictators who think they can win by making everyone else lose are just wrong. Life is not a zero-sum game.
And it's an uneven, unsteady progress towards freedom because it's so counterintuitive, because the obvious thing is that if I take something from you then I have gained and you have lost, when in fact we have both lost: the benefits of what I might have made if I had not been spending my energy taking from you, or what you might have made if you did not redirect your energy towards defending from me. But bit by bit, as a race, we're figuring it out.