Slavery was definitely treated as an evil thing, but as an entrenched one. The adventure included in the original boxed set starts the PCs out as slaves being transported across the desert. The caravan gets attacked by elf raiders, which leads to the PCs being "freed"... right out in the middle of the desert and barely being able to get a waterskin each from the wreck.
Then you have the first separate adventure, Freedom, where the PCs are enslaved and forced to work on the Ziggurat in Tyr. The picture painted of slavery is not a pretty one. And at the culmination of the adventure, the former king of Tyr is killed (by NPCs), and slavery is abolished in Tyr. And this is the point where it gets a little complicated. Later material, like the Tyr sourcebook, shows that while this is overall good, it is not without its problems. For one thing, you now have a large destitute underclass of former slaves, which is a destabilizing influence. You also have lots of resources (farmland, the iron mines) that are being abandoned because of the lack of workers (and the nobles don't really have the resources to employ their former slaves). You also have other city-states who see what happened in Tyr, and decide that they will not be having any of that, leading to a short-lived war between Tyr and the closest city-state, Urik (with the result being that Tyr defeats Urik in the field, but when they try to follow their success up and attack Urik itself in order to free their slaves as well, Tyr gets soundly routed). So the abolition of slavery is described as a positive, with an asterisk.