I think people like their favorite faction, but not everyone is automatically super bitter and full of hatred for the other faction.
I usually prefer the Horde, but I don’t really care about “defeating the Alliance”, I just want to see the Horde do cool things.
I suspect a lot of people feel this way, because it’s the difference between the well-recieved launch cinematic and Warbringers: Sylvanas.
In the launch, you see all the races doing cool things, but you usually don’t see the people they’re hitting, and when you do, it’s typical either very brief, or not the focus of the frame (and usually a bit blurry).
You see a Tauren charge through a barricade and several soldiers, but the camera is focused strongly on the Tauren, not the things in his way.
You see Dwarves and Night Elves both shooting, but you don’t see their targets.
Etc.
You’re seeing the races be cool and doing cool things, and you’re thinking to yourself “I want to make a [race] [class], now!”
By contrast, you don’t see anyone do anything cool in Warbringers: Sylvanas. You see catapults shooting, but you don’t even see their pilots, so they may as well be faceless machines. And, honestly, they’re not even cool by those standards. Compare them to the Iron Star Of WoD. The camera focuses on the Iron Star - something you should be very familiar with the power of, if you raided SoO - and the music grows tenser as it spins faster. The catapults are just there long enough to fire in Warbringers, that’s it.
There’s nothing really fun about Warbringers because it’s just 100% about seeing one faction get hurt, not seeing anyone on either faction do anything cool. It’s something that could work in a story, perhaps, but it’s not really meant for the narrative of a video game. A book or movie is supposed to evoke emotions in you, but a video game needs to make you invested and excited, it needs to make you want to do the things you see. It needs to motivate you.
I guess you could argue it succeeded since it motivated raids of Alliance players to run non-stop raids on the Horde capitals for a few days, but that’s pretty short term. As soon as that rage passes, it’s just going to turn to bitterness if they don’t find other ways to motivate Alliance players. Not to mention the division it creates out of game.
I can only hope Blizzard realizes how much they are shooting themselves in the foot in the long run by doing this.