Poul and Karen Anderson's Romans
Poul Anderson's Terrans and Merseians
SM Stirling's human and New Race Draka
(There are many others, of course, but I am just considering these examples.)
Is slavery wrong? It is certainly wrong to enslave prisoners instead of merely detaining them. However, there were past periods when slavery was the stage of development of the means and relations of production. A good Roman citizen could not overthrow slave-owning society but could treat his slaves well and arrange for their eventual manumission.
Legally limited "slavery" in Anderson's Terran Empire is more like a form of community service by offenders. See here. However, women can be sold to be used against their will as concubines, which cannot be right.
The Merseians are racial supremacists and we cannot regard their enslavement of other races as in any sense right. Human Draka enjoy imposing their will on others and should be resisted in every way. A Draka woman believes that it is right that serfs serve the Race and that she herself lives rightly if she treats obedient serfs kindly - whereas any who resist should be impaled. Aristocratic arrogance. But what would convince her that she was wrong? If:
(i) the Draka were to be overthrown by a slave rebellion;
(ii) it could be shown that the rebellion had arisen inevitably from the conditions of servitude -
- then she would have to accept that the Draka way of life had failed. But SM Stirling does not show this happening. Thus, for the Draka, Might continues to be Right. In the Final Society, the New Race is a different species, which changes the situation to some extent. The scent of Homo servus brings Gwen "...a warm pleasurable feeling, a desire to protect and guide." (Drakon, p. 9)
Thus, she no longer has the human Draka pride of Race, derived from imposing the Will on other human beings against theirs. I will have to think more about the New Race.