In one of my campaigns, set in a modified Mulhorand/Unther, the party, led by a priestess of Mulhorand, engages in slavery frequently. They sometimes don't kill or execute opponents, but enslave them. The priestess also buys slaves for her temple. I don't see the campaign as evil, rather as amoral.
In any case, I don't count slavery as evil per se in my games - at least not as more evil than absolute monarchy, feudalism with serfs, and corporal punishment (death penalty, mutilation, branding etc.) for minor crimes.
I have to add that modern values are not common in my campaigns anyway. My knights and noble NPCs are not egalitarian, they defend their "divine right" to rule over commoners with the sword if needed.