Lol... She could still win NP... IF she didn't say what she said. Probably provoked/short fuse/or whatever....
BUT.... what was her main trigger to just vanish away (in my opinion at least) - she still cares for the forsaken
https://www.youtube.com/watch?v=WX_oLGL7MoQ
4:12 kinda gives that vibe... how she looked at the flag barer... I could be wrong too, but it just gives me that vibe. She still cares for the forsaken which can be seen in her video (post-duel).
why others didn't interfere? just because they KNOW there would be MANY casualties. again, I repeat what Alleria said at Razor Hill about Sylvanas. That shes the only 1 capable of gathering army big enough to comfort N'Zoth.