While there were seeming preexisting issues upon which it was built, what was initially just uncertainty and distrust turned into extreme xenophobia in the Scarlet Crusade due to Balnazzar’s influence, whereby the Crusaders became convinced that anyone not of their order - and especially non-humans - were all carriers of the Plague of Undeath.
Which was ironically the opposite of the case; the Plague of Undeath was actually only engineered to infect humanity, and had a harder time taking hold in non-human species, usually causing overt and visually obvious mutations without actually killing such victims. Hence the Plague cauldrons causing grotesquely altered - but still living - plants and animals throughout the Plaguelands.
Ultimately Balnazzar’s reason for encouraging such a mindset was probably to insulate the Scarlets against outsiders with differing perspectives whose broader knowledge and experience in the world might undermine his control or even recognize his manipulations for what they were and expose him as an imposter.
Arguably some other racially exclusive groups that exist could include the Amani and Gurubashi trolls; while there have been times in the past that the former worked with the Horde, circumstances since have them basically framed in WoW as considering anyone who’s not a forest troll their enemy. Plus when they aren’t banding together to attack everyone else, the disparate forest and jungle tribes don’t really even like with each other. Only the Drakkari seemed to be relatively unified in purpose without a common threat, and have been shown to have coexisted peacefully with non-trolls living nearby (namely the wolvar.)
While there’s a sort of formalized historical exception concerning trolls, besides the Rajani the mogu clans have a pretty generalized belief that any non-mogu races are by default inferior fodder fit only for death, enslavement and/or harvesting as resources for their magics. In similar fashion the native races of Draenor have usually been prone to either xenophobic avoidance of racial outsiders or treating anyone different as food, slaves or both, with orcs being notable for breaking with this standard in recent history when they became the Horde, and ogres on both Draenors ending up forced to cooperate with non-ogres while basically under threat.
Therazane’s initial negative attitude toward the players was kinda two-fold; she’s angry about Theradras’ death at mortal hands, but there’s also a more broad sense that the inhabitants of Deepholm aren’t keen on non-elemental life intruding upon their realm and potentially spreading organic contamination. Which in fairness could be a legit practical concern, as the fungal overgrowth running amok in parts of the zone is attributed to the compromised World Pillar allowing spores to transmit over from Azeroth.
Something to keep in mind is that while the word has become common vernacular in fantasy universe parlance, “race” isn’t really the proper word for such a purpose, so transmitting ideas like “racial supremacy” from IRL isn’t really a natural comparison. They’re basically different species, and even when groups in WoW do hate outsiders who are of the same species as they, it’s virtually always along political, tribal, factional or clan lines rather than what would be considered racial differences IRL. While one might theoretically speculate that any hostility between forest, jungle and ice trolls might count as racially motivated due to the physical differences between them, such hostility is minimally represented due to their geographical separation, and perhaps notably it’s far less prevalent than just the hatred between any two forest or two jungle tribes fighting over the same territory.
Admittedly there does seem to be a streak of supremacist attitude among the Zandalari in relation to the other trolls, but it only manifests as hostility when one of the other tribes becomes violently aggressive against the Zandalari (usually for internal “doing bad magical stuff” reasons), and doesn’t really outlast the cessation of those hostilities.