For her actions in life, Sylvanas should have been rewarded greatly, she gave all she could to her people. She was a very good she elf. This needs to be rewarded. Our actions in life matter
In undeath however, Sylvanas is different, warped by dark powers, she against her free will, but she does so much evil too, funnily enough her motives are sometimes noble and in her own twisted way she is trying to do a good thing. However this Sylvanas should be utterly destroyed and severely punished.
What is the one thing that simultaneously achieves both?
Imo: it would be returning Sylvanas back to her original life or some form of modified Kyrian. It destroys the undead Sylvanas and as a big hater of life, inflicts the most severe punishment that Sylvanas could have wanted. Yet on the flip side, it gives back original Sylvanas a chance to live the life or if new Kyrian ensures a future that rewards her deeds in life..
Undead Sylvanas would utterly despise becoming a Kyrian or returning to being a high elf.. but the person that is reborn or restored would greatly appreciate and love that.