[Pursuivant]

Depending on how much the GM wants to emphasize it, and the players want to deal with it, slavery could be just a background feature or the focus of an entire campaign.

If the players are up for it, dealing with topics that modern Westernized folk find uncomfortable, but which aliens/historical people/whatever accept as a matter of course could be an interesting exercise in roleplaying.

For characters with disads like Code of Honor, Honesty, or Sense of Duty, attitudes towards slavery and the like will be heavily influenced by culture. For example, a 21st century Western European or North American transported to 1st century Imperial Rome is likely to be horrified by the pervasive slavery, sexism, cruelty, and social inequity. OTOH, a 1st century Roman transported to early 21st century America might be horrified by the absence of those things!

In societies where slavery is legal, a slaver who deals in legally purchased slaves, and who is generally peaceful and law-abiding, enjoys the same protections his society affords to other businessmen. Slaves who attempt to escape from a law-abiding slaver are likely to be in violation of the law, as might people who aid their escape. Slaves who fight to free themselves are guilty of assault or worse. Likewise, people who attack the slaver in order to free his slaves are potentially guilty of robbery.

That said, the line between slavers and kidnappers was often very thin. If a slaver attempts to kidnap a free person, especially someone who enjoys all the benefits of citizenship in that society, the slaver is guilty of a serious crime unless he can demonstrate a legal right to enslave his victim. In such cases, attacking slavers in self-defense, or the defense of others, might be justified under the law. But, just like any other use of violence for self-defense, the courts get the final say.