This is a pretty touchy subject, but here is my two copper on the issue. Slavery isn't going to be "Good" under any circumstance, since it impedes the freedom of others and even at its best can be somewhat degrading and dehumanizing.
Slavery is likely to be, but not always Evil, since it can easily lead to abuses and degrading treatment of the slaves. If the slaves are treated humanely, like one would treat any free man, except they are not free to leave their job, then I would call it Neutral on the moral axis, especially if the slaves had certain rights and protections (making them more akin to indentured servants from colonial history).
Slavery, however is also usually a Lawful act. In a typical fantasy setting it is a a definite and strong social structure, that in many civilizations acts as an underpinning element, by providing a working class that does all the labor. Perpetuating slavery would be Lawful since it upholds the expectations of society. If slavery is illegal in the setting, but it is still practiced, like providing an illicit labor or military force (or even more unsavory uses), then slavery is a chaotic act.
Thus, if the slaves are relatively well treated and legally protected in a society where their labor is needed and supports the general social fabric, I'd call it a Lawful Neutral practice. Example: 15th Century Mamluks in Egypt.
If the slaves are poorly treated, abused, and treated like livestock in a society where their toil supports a rich aristocracy who benefits from their sacrifices, it would be a Lawful Evil practice. Example: Early 19th Century American slavery.
If the slaves are abused and poorly treated in a society where slavery is legal, but not widely used and only benefits a small group who opt to use slaves, I'd call it a Neutral Evil practice. Example: Most slaves in the Star Wars universe.
If the slaves are abused and treated like livestock in a culture where slavery is illegal, but the slavers persist, presumably to use the slaves as disposable labor, shock troops, or for raw sadism and decadence, I would say it would be a Chaotic Evil practice. Example: Modern day "white slavery".
If there happened to be a society where slavery was illegal, but a group practiced it as a means to get a labor force, but treated it's laborers well, it could be considered Chaotic Neutral. Example: A thieves guild in a fantasy kingdom that bans slavery, but the guild takes orphans and urchins off the street and presumably a short life expectancy and offers them food and board in exchange they work for the rest of their lives for the guild as messengers, servants, and informants.
If somehow there was a society where slavery was not illegal, but rare and legally tolerated, and slaves were treated relatively well, perhaps even enjoying legal protections, I could call it a True Neutral practice. Example: A fantasy setting where slavery is technically legal as a long forgotten law, a young beggar tries to pick-pocket a member of the PC party and is caught, and while facing execution, the PC's realize that under an obscure law they can insist that the beggar be enslaved to them instead, and he must be treated humanely but he cannot be legally emancipated for "X" years, so a PC gains a follower/cohort that literally and legally owes the PC his life.
