More or less what Droite said.
A few things, fellow blue-posters. One- the Horde is a playable faction. Full of half of the game’s player base. Remember how crappy Cata felt for the Alliance, where the emotional impact of the content was “Alliance loses all the time”?
That shouldn’t be how any faction feels, at least, not against its counter parts.
Second, the only way this works is if the script gets flipped. The Alliance commits some big, actual atrocity. Not some “that one thing they did in an intro quest that is never relevant beyond it.”, Not “that one minor camp that was destroyed in story off screen in the midst of a total world revamp.”, Not that “Time Jaina did some stuff, but it is confined to the quest itself, and otherwise lost in the sea of the next psychotic thing the Lich Queen is doing.”
If it were to be done, it would have to be the Alliance, clearly, doing it, the PC would have to be involved, and it would have to remind players of it every single time the place it happens is referenced. Theramore. Teldrassil. The Vale of Eternal Blossoms. Gilneas. Not that it was even current horde at the time, but Netherwind.
Third, for the rest of the Xpac, the Alliance gets reminded about how evil that action was. Furthermore, everything the Horde does for the expansion is going to be stacked against the atrocity, and thus justified to an extent. Horde kills some perpetrators with a mess of collateral damage, raises a few as Forsaken? Tough cookies, Alliance did the big atrocity. Horde has a raid in the Alliance’s Xpac capital and kills a new NPC leader we just got? Alliance had it coming. Good Alliance NPCs turning on whatever characters support or were involved in the attack and work with the Horde? That’s what a hero does. The game gets framed in the context of the atrocity, and everything to some extent revolves around it.
Finally, and this one is important- I know many of you feel the Alliance gets the sharp end when it comes to consequences, right? And one of the expressed frustrations is that “Horde do bad thing, Alliance do random kinda meh thing, stuff happen, NPC says a thing, and no consequence.” So, do you really think the Alliance would get the same treatment? If you truly believe Blizz to have particular proclivities regarding how they treat the factions, do you think it would end with Lor’themar telling the collected Alliance leaders that if they are bad, the Horde will end them, and their cities intact?
Or would it end up with a corpse pile, a city with long lasting physical damage, and possibly NPCs free-willed into zombie servitude?
Because there’s no rule that the reverse would play out the same way, after all.