I mean, I agree, I'm super surprised that she just burns the tree, but I read on another forum that it's something to do with the fact that the elf made her realise that since malfurion got away, the tree cannot stand or it will be a symbol of hope. The symbol Sylvanas never got when arthas invaded, and now she will have to do the same to the alliance to protect her people (and herself of course). Idk. It's still basically evil but I can see it being within her character and in her strategy.