This seems a weak argument.
You can't show that imprisonment as a mode of punishment is morally wrong simply because some prisoners try to break out;
I wasn't even trying to do that, so... thank you for the strawman.
I was responding to the idea that rape, murder, and slavery become Good when the culture in question treats them as "acceptable".
But to show that slavery is a wrong you probably have to more than show it causes suffering. You have to show that it does so unjustly.
No, I don't, on several levels. The most basic being that you appear to have some delineation between "wrong" and "unjust", and causal logic from one to the other, that hasn't been given to the rest of the class. You can't expect folks to try to prove anything when they don't know the underlying reasoning and definitions you are using - that way lies at best misunderstanding, and at worst moving goalposts. So, I'm not going there.
But, that's a tangent, as you are demonstrating again how D&D alignment should be scrapped by way of it not getting the point across.
In the real world, words like "good" and "justice" are not subject to proof. They are
opinions. I may be able to convince you to agree that slavery is wrong in any particular case, or even that slavery is wrong in all cases, but that agreement is as best consensus, not proof in any useful meaning of the word.
Aside: if we were to try to do this, the best we could manage would be to show that treating slavery as "just" at any time leads to cases where the system is inconsistent, but we are by no means guaranteed to agree upon whether that inconsistency is acceptable.
In the standard D&D world, again, it isn't subject to proof because, as you will likely remember form whatever studies of formal logic you may have had,
axioms are not subject to proof. The multiverse tells us whether it is wrong not, and does not allow debate on the topic. In the standard D&D cosmology, the creators of worlds, the gods themselves, are not above and beyond the metaphysics - they are bound to it. It is a given thing of the multiverse, not subject to external logic or argument.
But, here you are, one of the more rigourous debaters on the boards, speaking as if it were otherwise. Thinking that either of us could prove (or even the softer "demonstrate") whether a thing was right or wrong from outside the system, without knowing the base principles of the metaphysic, is a failure of the alignment system.