The difference is that you, the player, choose to do this in Stellaris. When I play Total War Warhammer, I'm perfectly fine choosing the Dark Elves and playing as a slaving, pillaging, torturing, murderous, quasi genocidal bunch of maniacs. Because there's whole lotta factions to choose from if I don't like this theme. When playing WCIII I favored the Undead. In Starcraft, the Zerg. In my latest playthtough of Dragon Age Origins I played a total freaking scumbag who murdered on a whim, massacred an entire species, sold his own people into slavery for power and ruthlessly played a political game at the expense of his friends. Because I could choose to do it.
In WoW, you're Alliance or Horde, which in BfA means you either follow a bunch of morally flawless people in perfect symbiosis with each other and whose every subject is a veritable paragon of virtue, or a bunch of savages led by a pillaging, torturing, team-killing, dead-raising, quasi genocidal sociopath. There's no middle ground, no choice if you want to play something else. You either follow a bunch of angels or Satan with tits. That's just bring as hell to me.