As far as societies in my campaign-world go; the great majority of societies have slavery, regarded as a fact of life in most areas. Some religions & philosophies disapprove of it to greater or lesser extent, and these are mostly of LG alignment - Thrinism, Northwoods Chrisism, Imgart Ukkoism. These areas all have feudal societies with little economic need for slavery. By contrast the 'democratic' (in the Athenian sense) NG/C creed of Cartheanism is primarily urban & non-feudal and has no problem with slavery at all. Most of the slaveholding societies IMC are essentially Neutral, whether civilised or barbarian. To be counted as Evil the society (human or nonhuman) has to be dedicated to an evil creed, typically an evil god that advocates genocide, human sacrifice, extreme oppression of a particular race, gender, religion, etc.