The trouble with Sylvanas, and this is something that I hope Blizzard will expand upon with her character, is that she has put herself in a box surrounded by potential treachery. Between her vicious forsaken encouraging her heartless attitude, whether directly or not, and the alliance creating an enemy out of her for accociating with the horde, she has to juggle the expectations of the horde, the forsaken and her enemies. It leaves little room for morality.
Also, let's acknowledge that until becoming the banshee queen, Sylvanas had practically no affiliations as a leader. Sure she was a general for the army, by she was never a diplomat or royalty. She assumed the role of Bashee Queen and succeeded only because her followers wanted the same thing she did.
I genuinely think that she may go insane, if hopefully temporarily, over all the responsibility she has been given and the expectations everyone has of her that she cannot meet. Sylvanas suffering depression and an existential crisis I feel would be a fascinating and dramatic way for her to begin reforming.
The thing with Sylvanas is the amoral at best operations of the Forsaken in using Scourge developed techniques to increase their population. These are only not evil if you are both Forsaken and think it is an existential requirement to maintain the Forsaken as a continuing concern. From almost every other perspective, it is just evil. Using the Plague of Undeath is evil unless the victim is already dead, but it's not clear if it would work on the already dead. As long as the Forsaken use the Plague to bolster their ranks, they are definitely evil, and that includes Sylvanas as long as she approves.