Here's a question that, no doubt, the lore team asks themselves constantly when designing new content; "How can we convince players that these creatures they need to kill are bad?" The lore team's go-to answer, for some time now, has been "fuckit, let's make them slavers,"—and it's become a trope that I'm pretty damn sick of. WoW has an incredible variety of races and monsters, many accompanied by their own fantastic lore. To homogenize them all by saying "hey, its okay to kill these guys, because they're slavers" is boring. Get players to want to dismantle the societies you want them to for more interesting reasons—maybe because this one's a future threat, maybe in this case WE'RE the aggressors, maybe in another case there's a border dispute or a trade disagreement or something. Slavery has become something that Blizzard just tacks on to societies to ensure that players have no qualms about absolutely dismantling them.
Here's a brief list of creatures/societies that often have slaves in WoW.
- Naga: Broken slaves in BC, Alliance/Horde Slaves in Cata.
- Twilight Cult: Slaves of all races in Cata.
- Ragnaros: Dark Iron slaves in Vanilla.
- Vrykrul: Enslave their own race in Brunnhildur.
- The Lich King: Enslaves anything that he kills, through resurrecting them as undead.
- Orcs: Enslave everything from dragons to Ogres. A notable exception is the orcs of the Horde.
- Magnataur: Enslave snowbolds. This is actually a more interesting relationship, since the snowbolds worship the magnataur, so this gets a pass.
- Mogu: The race that literally enslaved every other sentient race of Pandaria. In MoP, they have Tauren and NE slaves as well.
- Ogres: In WoD we find out that, guess what, the Ogre empires are built atop of slavery. One of the dungeons is literally based upon freeing slaves. One of the raids features a gladiator-boss. We might not be killing ogres just because they're slavers, but Blizz sure as hell is going to justify killing them because of the fact.
Notice that the Horde and the Alliance, or any of their allies, never, EVER have slaves. The closest they come, iirc, is when Captain Doran of the Alliance expedition to Pandaria 'forcibly conscripts' Pandaren to build an Alliance base in the Jade Forest, but he was going off the deep end, anyways; and when Okrilla enslaves the ogres of Dreadmaul Hold, but she goes over to the Iron Horde in WoD. The Alliance might occasionally destroy peaceful Tauren villages, and everything that the Forsaken do might be a war crime, but never do either enslave any sentient race. Even the arguably most evil playable society, the Forsaken, are based around the concept that slavery is wrong. Any undead that they resurrect has to option to not join the Forsaken, which you learn if you do the opening quests in Lordaeron.
Notice that slave societies are never redeemed—any peoples who enslave another are 'black-and-white-evil'. Never has one society convinced another to change their ways and free their slaves. Instead, the right thing to do is to destroy slave societies, because obviously they're all irredeemably evil.
Anyways, my point is that slavery is quite frankly an overused trope in WoW. I wish Blizzard's lore team would think up more imaginative reasons to justify killing stuff besides that "HEY THESE GUYS HAVE SLAVES AND THAT'S BAD SO OBVIOUSLY DON'T FEEL GUILT WHEN YOUS KILLS THEM". I'm not asking for WoW to becomes as 'shades of gray' as Game of Thrones (but christ would I love that), but I am asking that it not stay as black and white as LoTR.