First let me say that I've predicted the burning of the Nelf tree wouldn't be by her hand. Imagine my surprise yesterday so yes. I could be wrong assuming the following:
1. She becomes leader of the Horde because Vol'jin says the Loa chose her. Ok, didn't seem like a big deal at the time but now we go to Loa land so I suppose that has to be taken under consideration.
2. The biggest problem right now are the Void Lords, or so are we led to believe. Alleria who is pretty much their fiber optic cable into Azeroth gets some really urgent messages about killing Sylvanas on the spot. So, I'm guessing if they want her dead so badly then there's more to this story.
3. The blighted corpses in Ashenvale. Last time I did that quest it was a while ago on the beta but the corpses were blighted. But there is no blight, just the player and a crazy belf assassin. So unless they changed it and Sylvanas blights the town, there's something else happening.
4. The act itself. Buring the tree. Why? And this is something that bothered a lot of her fans who are fully aware that she's evil, not morally gray, but evil. But why? Why burn the tree? Why not just do something even more gruesome instead. I mean she had the people inside at her mercy she pretty much could have done anything she wanted by that point. Why burn the tree?
If you have your own theories about this situation then let's hear them. Is there a polt twist somewhere? We are being intentionally misled by the writers so we can get our minds blown with the unveiling of the whole story or is this just another Warchief execution a'la MoP?
Kindly try to leave your "faction pride" at the door and analyze this as someone reading a story and not someone looking for a forum war.