Well, maybe it's not Horde or Alliance they care about, but to fight Elune, who let it all happen. Pure hatred, like Delaryn had at the end of Elegy, when she cursed Elune and renounced her loyalty to her. Just like Sylvanas wanted.
Edit: In A Good War, there's a part where Sylvanas is happy about the unnecessary cruelty some of the Nightelves inflicted upon the Horde forces, because that's what war makes of you. She likes it, it's what she wants. And maybe it's also why they then follow her, because they are as hate-filled as she is and want nothing more than to inflict their own suffering on the world. Oh and maybe because they now share her end-goal, whatever that may be. The death of Azeroth, the death of the cruel gods, the death of everything. I guess we'll see soon enough.
Edit2: And may I remind everyone (who cares to be reminded
) of the questline back in Vanilla where the Forsaken used their plague on a barrow den in Ashenvale? You did not see it from a Horde perspective, so it wasn't that you found out what they actually tried there apart from testing their plague like usual. It's the place that is now full of Forsaken Assassins and ghost druids, though the guy giving you the quest to kill them nowadays says the druids were raised when the Horde corrupted the Heart of the Forest. Dor'danil Barrow Den.