Her intention was to burn the tree, but that would have come later, after the Alliance had lashed out in desperation. A Good War actually alludes to this with one particular line after Sylvanas learns that Malfurion survived. I’ll quote the entire passage and bold the important bit.
This battle was not about a piece of land. Even Saurfang knew that. Taking the World Tree was a way to inflict a wound that could never heal. Losing their homes and their leaders would have ended the kaldorei as a nation, if not a people. Even the loss of one leader would have been enough to create a tide of despair. The wounds of this battle would have bled, festered, decayed, and rotted the Alliance from the inside out. Anduin Wrynn would have lashed out in a final, desperate war, looking for a miracle, because only a miracle would save them.
But a miracle already had. A miracle granted by the honorable hand of a foolish old orc.
And an overconfident warchief. Best to lay blame where it belonged. This was her mistake as much as Saurfang’s.
This conquest of Darnassus would rattle the kaldorei people. They would grieve for their lost, fear for their imprisoned, and tremble at the thought of the Horde ransacking their homes. But they would not fall to despair. Not anymore. Malfurion’s impossible survival would give them hope. Their wound would heal.
Even in this dark hour , they would say, Elune still watches over us.
And that was almost certainly true, wasn’t it? Elune had intervened. Perhaps she had even stayed Saurfang’s killing blow. And she wouldn’t be the only force beyond the Alliance to oppose Sylvanas’s true objective.
Her true objective. To feed the Jailer souls. She would have used the conquest of Darnassus as the starting point, then after the Alliance had rashly engaged her forces, she would have executed the Night Elves and set the tree alight. That was always the plan.
But after Saurfang failed, she realized that she had to change those plans. Now she couldn’t rely on the Night Elves to collapse into despair, so rather than burn the tree later, she shifted her plans around and burned the tree first.