I've been very entertained by this discussion, since I've been attempting to address exactly this point in my next campaign setting.
To funsangite's point, about playing in a pre-modern mindset, I'm explicitly touching on the concepts of 'equality', 'freedom', 'vote', and 'slavery' in the set-up / first session, and reminding the players how their characters world-views are different from their own on those American ideals.
The thing which stands out in reading this debate is that some definitions are missing.
I keep hearing 'slavery is evil', but when I read the posts more closely, I find that even those people writing it are posting things like
That somehow taking away another person's freedom and declaring them your property is not utterly and completely evil.
And sure, I think the people on the 'slavery can be neutral' side would agree with that: forcibly taking away somebody's freedom and declaring them my property is clearly an evil act. The paladin doesn't march into the enemy camp, defeat their leader, and claim 'Now you all are my chattels, and must do my bidding.'
The flip side, those who are arguing 'neutral' can argue that there are models of slavery in which the slave is not forced into slavery; has rights and the possibility of freedom at some point; etc.
This comes down to 'how do you define slavery', and 'how do you define oppression'; I think that it is possible (and in fact, has been done in these six pages) to imagine a definition of 'slavery' and a definition of 'oppression' in which 'slavery' does not map to 'oppression', and furthermore doesn't give the 'slave' one-hundred-percent 'free will'. I haven't seen anybody argue that slavery as practiced in the American South wasn't an evil institution; just that it is possible to imagine a non-evil institution which meets a definition of 'slavery'.
For some fictional reading material which touches on a slavery institution in a fantasy setting, I reccommend Alanna Morland's works,
Leopard Lord and
Shackle and Sword; there are some interesting examinations of master/slave relationships in a world which includes the moral absolutism of good and evil.
Lastly, in my campaign (personally!) my intent is to depict a feudal / heriarchal system with a prevalent good/evil moral absolutism. In this system those higher on the social scale may be perceived to have more 'rights' but are also coupled with more 'obligations'; for an example touching on slavery, your 'good' paladin would not be shocked to discover household servants in the manor of some lord; however, if the lord were mistreating them in any way (failing to uphold a number of social contracts: food, shelter, justice, compassion), the paladin would be convinced that she had seen an evil act. I'm imagining 'entry to slavery' via some of the discussed neutral means: voluntarily entering into an indentured servitude contract; accepting a (limited) period of servitude as an alternative to other less desirable punishments for a crime. I'm struggling somewhat with the details of 'bonded servitude', but am imagining a system in which prisoners of war are typically ransomed by their families (if wealthy enough) or lord (as part of a peace settlement), but that entering a period of 'bonded servitude' exists as a means for prisoners not otherwise ransomed to earn their freedom.
Is it 'good'? No; however, I'm envisioning it as a 'neutral' establishment with the potential for abuse. I don't want my players to feel that their characters need to overthrow the 'evil oppressive monarchy system' to bring an enlightened democracy to all of the world's peoples, nor do they need to go through a five-year-long war to emancipate all of the enslaved peoples of their civilization... but it leaves me plenty of room to for them to encounter a culture which treats slavery oppressively, denying the slaves the rights which they expect slaves have and in which the slave-holders are ignoring their duties to their slaves, and to paint that culture as
evil beyond compromise.