People taking the Prologue of the novel to heart have to consider that it's the prologue.
There's a reason they released it publically, and honestly, I think they intended it to be a bit misleading.
I say the novel prologue is a big ruse, and while we know it is Sylvanas burning Teldrassil with Sargeras' sword thanks to the leaker, I doubt she did it for an "evil" reason.