Originally Posted by
Aerisot
That is not at all why she did it, however the nightelf did give her the motivation, She only wanted to kill Malfurion to make all nightelves lose hope, but when Saurfang refused to end it and let him go, she needed to switch gears, and she realized that just taking over the tree was not good enough, it would not stop the nightelfs from having hope, so she took the one thing sacred to them...and not just the tree...I'm talking about their immortality.
In one fel swoop, she extremely lowered the population of her enemies, and also caused the ones that survived to lose their immortality, including Malfurion who is basically on deaths door right now.