A hero, sure. But she was still an undead. Again most normal high elven civilians would not be okay with that. Just look at the humans. They didn’t have nearly the same struggle with the undead and they all despise the forsaken. The High Elves have even more reason to want them all wiped out of existence.
I mean, sure. The High Elves are a pretty selfish race, and they don’t work well with anyone other than their own kind. Even in the horde it’s considered an alliance of convenience. Still, most high elves would work with the alliance over the horde any time. The horde is composed of all their greatest enemies. And the High Elves have been known to hold grudges. Again it makes no sense for them to be horde other than just wanting to give horde a pretty race which is exactly why it happened.
If anything they would have stayed independent.