How do we handle slavery in our campaign? It depends on the campaign, and the PCs.
In one campaign, where the PCs are by and large good, with a paladin a prominent member, and have experienced slavery first-hand, slavers are prime targets. That group will, when possible, free slaves and fight slavers. Still, in entire societies that practise slavery the group will be able to function, as long as their butons are not pushed, i.e. no cruel treatment is encountered, for as long as their mission may take.
Now the other group practises slavery. Several of the PCs own slaves, have recently enslaved a captured foe instead of killing her, and would defend their society (and slavery with it) against all foes, even paladins battling slavery. They did, in fact, foil the plans of a group of adventurers that was trying to free a captured and enslaved member. Here the PCs are more or less neutral, some with a darker side, some with evil tendencies and some outright evil. Still, they do not mistreat slaves, nor use them as cannon fodder or trap detectors - not only because that would be foolishly wasting your property, but also because they have enough "good" left in them.
As a DM, I try to treat slavery as it was historically practised by the Romans, the greeks, and other peoples, and I also have serfdom, which is common in many feudal societies and basically slavery by another name.
As an aside, I do not try to make my world a better place than historical earth, and truly good individuals are scarce, and people who are good and share our modern morals almost non-existent.
IMC, even a LG person will see usually see nothing wrong with the death penalty for many crimes, with killing your enemies, and with living in a caste- or class-based society. Power will corrupt, and people will abuse their power. Nobles may have a ius prima nocte, of "Braveheart" infamy, be premitted to kill offending peasants without trial, and their words may have more weight in a court than those of commoners.
In that context, I treat slavery, rape and war as they were and are practised in our world.