It was killing Malfurion. That was her secret plan, all along. It wasn't "A war against Life" or something similar. She was hunting his ass down in order to inflict "A wound that would never heal".
She didn't tell Saurfang her plan to kill Malfurion during the planning stages of the War of Thorns. Only that they had to inflict that terrible wound. Saurfang thought that taking the tree would create that wound, but it was only after the tree burned that he understood that keeping the tree was just a bonus for the Horde War Plan, and that Malfurion's Death was the real goal. Because only his death would push Tyrande into trying to reclaim Darnassus while Anduin and Genn did their best to hold her back because they didn't have the navy to support it. That the Alliance would tear itself apart over their separate interests.
But that would only work if Kalimdor were "secure", and Malfurion was dead.
She doesn't have some far-reaching hidden agenda that we don't know about, yet. She laid it all out for Saurfang: The conquest of the Alliance.
And it's also why she burned the tree... with Malfurion alive, she needed a new "Wound That Would Never Heal". And Teldrassil and it's civilians were it. She didn't kill them out of hatred, or spite, or sadistic cruelty that warmed her frozen heart. She did it because it was the only way to get the Alliance to attack Lordaeron, to get them to fall into the trap and decimate their military forces so that the defacto Peace of Distance could hold.
Aside from Saurfang brooding in a cell in Stormwind, and Jaina saving Anduin, the plan has gone like it's supposed to.
And that's fucking terrifying.