This is incorrect.
To quote an older post of mine on why Dark Rangers/Wardens so willingly serve Sylanas:
To understand why it makes sense for these characters to so loyally serve Sylvanas and her Horde, we need to go a little bit back, back to the Burning of Teldrassil. During the Burning of Teldrassil, as explained before, the Night Elves were genocided. It is stated in Elegy that the numbers of the kaldorei remaining in the world are “few, too few”.
The Night Elves have revered Elune for millenia. But when these loyal worshipers needed her aid, her protection, Elune left the kaldorei on their own. Teldrassil burned, and many kaldorei died. After revering her for so long, one may begin to question their faith in this deity – why didn’t Elune intervene? Why didn’t she save her worshipers?
It is also stated in the Horde introduction that both Sira Moonwarden and Delaryn Summermoon believe that Tyrande and Elune left the kaldorei to die. They abandoned their people and left them to die. Delaryn Summermoon states that she is forsaken.
Add to that that the curse of Undeath brings with it its own mental side-effects, and you can very easily imagine that Sira and Delaryn would willingly fight against those that they believe abandoned their people, Tyrande and Elune. The Dark Wardens and Rangers symbolize the despair of the Night Elves in their times of need.
TL;DR:
Teldrassil burns, the Night Elves face a massive loss, some Sentinels begin to doubt their leadership/their faith & believe Elune and Tyrande failed them. Added mental side-effects of being raised and there is your reason.
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