The funny part is she didn't even want to burn the tree lmao. She was followed to do these actions, by seasoned veterans mind you, as retaliation for what happened. The whole basis of the Horde army that marched on Ashenvale and Teldrassil was "going to silithus" originally to protect the miners that the Alliance had been murdering, with zero justification, and that's the story the Alliance knew. That's why the Night Elves were caught off guard because they sent ships down to Silithus to counter the Horde army. Being caught off guard =/= genocide. It's war and the Alliance made a decision based on the information they had, it was a bad decision, and it cost the Alliance a capital city. As far as 'canon' goes the alliance calls it genocide, it wasn't genocide by definition, that's not up for debate at all because it doesn't fit the definition of what genocide is.
All that aside, if it wasn't for the Night Elf hubris, they had plenty of time to evacuate and chose not to because they were arrogant.