Not so.
He is enraged, but he also directly questions her. He listens to her. Then admits she is right. Again.
He does not just become paralyzed.
He admits she was right. He recognizes it was his plan from the start. He recognizes killing hope as part of the plan that he threw away:
He struggled to form words. Finally, pure hatred made him spit out a condemnation. “You have damned the Horde for a thousand generations. All of us. And for what? For what?”
Her expression didn’t waver. “This was your battle. Your strategy. And your failure. Darnassus was never the prize. It was a wedge that would split the Alliance apart. It was the weapon that would destroy hope. And you, my master strategist, gave that up to spare an enemy you defeated. I have taken it back. When they come for us, they will do so in pain, not in glory. That may be our only chance at victory now.”
He wanted to kill her. He wanted to declare mak’gora and spill her blood in front of Horde and Alliance alike. But she was right.
A wound that can never heal. That had always been the plan. And Saurfang had failed to inflict it. The story of Malfurion’s miraculous survival would have spread among the armies of the Alliance as proof that they were blessed in their cause.
War would still have come. That had been certain the moment Saurfang had led the Horde into Ashenvale. And it would have been what he had feared most: the meat grinder, spending so many lives to achieve so little, ending with a whimper, and thus dooming future generations to a war nobody could win.
Once again, Sylvanas had seen it before he had.
After a chat, he admits to himself that Sylvanas was right. Saurfang recognizes that the wound that will never heal in order to destroy hope was part of the plan to begin with. He recognizes sparing Malfurion went against that plan. And he recognizes the fact that he threw that plan out the window.
I think that does well to explain some of why Sylvanas changed her mind. The goal was to break their spirit into submission, not simply holding the tree. That goal was lost, since Malfurion’s survival would be seen as a miracle by the Night Elves, even if the tree was held by the Horde. Now, the only chance the Horde had was if the Alliance made a stupid mistake. And she had to shake them to their core to get them to make a mistake.
A few pages before she orders the tree burned, the reader is informed that she is already thinking of something to do. But she does not know what to do. It is not like she just flipped and burned the tree out of madness. She was considering what great wound she could cause that would measure up right after she says “Secure the beach.” :
“Secure the beach,” Sylvanas said. “Prepare to invade the tree.”
A wound that cannot heal. Sylvanas needed to think of a new way to inflict one. There was no turning back.
“Why?”
The voice drew Sylvanas’s attention from the tree. It had come from a mortally wounded Sentinel, the very one Sylvanas had felled only minutes ago. She was coughing. Weak. Dying.
“Why? You’ve already won,” the night elf said, struggling to force the words out. “Only innocents remain in the tree.”
That was good to know, if it was true.
Sylvanas knelt next to her. “This is war,” she said.
I think this shows that the Burning of Teldrassil was an on the spot example of a change of plans because of thoughtful consideration.
This passage seems to show the burning of Teldrassil was not the plan from the start, or a sudden emotional reaction, imo. But something Sylvanas came up with while brainstorming what sort of “wound” she could make. Which is more a consideration than an outburst.
