The Sylvanas book is seems to be attempting to set up that even as a child she resorted to harming others. That vengeance was always her first resort.
First thid of the story done and Lirath is still alive and Nathanos just showed up (If this becomes Twilight Romance Story I may puke).
Halfway through and now Vereesa supposedly caused Lirath's death by training him to fight and Sylvanas has to be restrained from hurting and/or killing her. (I do not know the lore enough to know if this is a retcon) Lore'thmar appears to have realzied Sylvanas is unstable.
They really are apt to make it that she was always crazy. They have her poisoning her own troops now. And there we go shes fucking Nathanos now....Maybe Danuser wrote this and not Golden.
And now Arthas has her slaughtering children and most of silvermoon.
They now have her returning to her friends after breaking free and hearing that they think shes a monster and that shes pretty much at fault for all Arthas made her do.
They are playing that Sylvanas has no ability to feel good emotions as an undead. They also make constant comments that her physical body feels like its not her own nor a part of her.
She has just molded Nathanos back into her comrade, because she feels no one but him will abandon her.
They are also playing that she never felt anything for her people:
"Everyone, everything, was a weapon, now, to be aimed and released wherever she so chose. Certainly the Horde and its leaders, but even the Forsaken themselves, for Sylvanas Windrunner knew three things: They loved her, and that love was as pure and true as creatures such as they could offer. And the last truth, one she could never reveal, was that any semblance of compassion she felt for them depended entirely on their continued usefulness to her ambitions."
Now she fully supported the torturing of the Forsaken for the blight.
The Jailer made her beleive that Frostmourne was used to bind his will as well and between moments of torture and agony he would give her glimpses of hop in joining him to end it. She also realizes that he in essence is Frostmourne itself.
So the Jailer pretty much tortures her so that she will agree and beleive him. The Valk'yr were sent by him to persuade her that he was truly offering her the ability to transcend the cycle of death.
He also tells her shes been deemed past any redemption and nothing will change that unless she joins him. When she asks the valkyr to show her the good of the shadowlands they only show her situations where people are stripped from their loved ones for all eternity. The torture of revendreath, etc.
The jailer informs her that she must remove all attachments to the world and only destroy, so she does.
He gave her prophecies of what would happen to prove his loyalty to her and as they came true she fell more into his spell.
The burning of Teredressail was by order of hte jailer to send more anima into the shadowlands.
It ends with her seeing Lirath in Andiun and that being why she was so hesitant to hurt him. When Anduiin calls this out to her she goes full on paranoid unhinged and makes the decision to force the mournblade.
And so ends Sylvanas's fate:
"Sylvanas had turned to the night elf leader, Tyrande Whisperwind, for her judgment, because she knew no matter how cruel it was, Tyrande’s punishment would be just. And so it was. Tyrande, rigid with rage tightly leashed, had led Sylvanas to the edge of the Maw. It was in this place, Tyrande decreed, that her penance would begin. Every soul lost in its depths, betrayed or condemned, you shall find and send forth to the Arbiter. Both women knew the task was all but impossible. Souls had been entering the Maw since the very creation of the machine of death. Tyrande smiled coldly as she saw the realization on Sylvanas’s face. You will toil there, the high priestess had said, savoring the words, scouring every darkened reach, until the final soul is free, and you are all that remains. This is how you shall bring renewal to your victims, and my people."
Which means shes sure to return 10.0 all squeaky clean and shiny.