D&D takes place in the middle ages.
In those days slavery was common.
Good people could keep slaves and do things which by our standards would be evil.
Women and their situation is one example.
Women were essentially "machines for producing children" as Napoleon once said so it shouldn't come as a surprise that people would keep servants unwillingly if necessary.
Aristotle kept slaves and he was still considered a good man in his society.
This takes the thread into an alignment discussion which, based on my past experience at EnWorld, is unlikely to be productive. It's not that I don't agree with you per se, but people's real life moral positions are likely to make this a hot button topic.
First of all, you seem at first blush to be taking a stance of 'all morality is relative or subjective'. That might not be your intention, but from what you wrote I can't really tell that you aren't going that way. While that's one way to interpret the data, it's not the only possible way to interpret it. And, it's an interpretation that's likely to cause a real life alignment argument as the moral absolutists (lawful?) square off against the moral relativists (chaotic?), each with the feeling that if the other is not down right evil, then at least that they are certainly ignorant.
For example, it could be that Aristotle was good by the standards of his society, and this made him actually good. Or it could be that he merely lived up to the moral norms of a society which was objectively Lawful Evil (or some such), and so other Lawful Evil persons judged him to be 'a good man' in as much as he lived up to the tenents of Lawful Evil and most people in the society felt Lawful Evil was the morally correct choice.
Or, it could be that things such a feminism and slavery aren't actually in and of themselves things that have moral value (as we in our current society suppose), and that whether a society is slave holding or patriarchial is not sufficient to judge its moral character because those things in themselves don't hold moral weight. It could be then that some societies that held slaves were in fact Good, because they held beliefs such as a master had certain obligations to his servant, that it is was wrong for a master to be cruel to his servant and a man who did so was not fit for polite company and otherwise subject to social or legal penalties, that being a slave did not confer on a person a judgement of their moral worth as slavery was a condition anyone might unfortunately find themselves in and that being a good slave was a path to perhaps even high honor. Or the society might hold that slavery was a condition that one could only rightly hold a slave in for a certain period of time, or under a certain contract, and that after that time the slave (and all his offspring) were owed their freedom (if they desired it) and perhaps even certain wages. In this case it might be that a society holding those beliefs might have tamed the morally dangerous instution of slavery in such a way that the slave-holding society was good. Or it could be, that modern mainstream thought on the subject is correct and that simply holding slaves condemns a society to be judged Evil.
In the later case, you might view society as being overwhelmingly Evil over the course of its history, but that over time man has become more and more enlightened until at last truly Good societies have begun to spring up. Or conversely, if you don't hold this later view, you probably think that for all of modern's societies pretences of being good, whatever it has learned of goodness has been completely offset by what it has forgotten of goodness. Or you might even think that society is continually morally deteriorating.
The problem you are likely to run into is that EnWorld is almost certainly populated with people who hold each of these various politically charged positions, and it is by no means our place here to argue them out of it. Hense beyond stating the complexities involved, we are left with little to say.
Which is why I completely sidestepped the question by trying to show that even within the terms of the myth, it doesn't fit in to the moral framework of 'slavery' that a modern person unfamiliar with the details might suppose.
