It’s pretty much already been said, but yes it would be out of character for an alliance character to commit an atrocity on the scale of Teldrassil, and permanently stain that character and the faction itself.
This is also true for the Horde. Teldrassil was out of character and permanently staining itself. I can’t overstate how horrible of a decision narratively they made with the War of Thorns. It’s almost impossible to make it worse.
I know a lot of people hate Sylvanas and view her as evil, but just because someone is evil does not mean that EVERY evil act is in-character for them. She had a specific character, and deciding to randomly burn down a city full of exclusively civilians after starting a war for pretty much no reason does not fit it.
Anyways, my primary point is that currently I am not sure if the writers care if some is out-of-character or not, when determining if it will happen.