Sylvanas' morality is pretty much black by this point though. There is a level below for completely irredeemable character like Archimonde, but as far as evil characters go she is pretty far down there.
The comparision to Arthas is quite apt, the difference being that so far there is no good reason given for why Sylvanas went completely insane other than not wanting to go to hell. Arthas did have a motivation for good at the core that got layered in more and more evil acts as he went along all up until he picked up Frostmourne.
Sylvanas meanwhile has no such good explanation given as of this moment, and the magnitude of her evil acts means that her "good intention" has to be of such magnitude that her acts can somehow be somewhat excused, and given the acts we have seen, and her seeming refusal to confide in others what this intention is then the good intention has to be the defining moment of the entirety of SL for the playerbase to buy it.
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I would argue that we don't have enough time to know stuff about him. Especially not if we want his defeat to feel more important than the final death or defeat of Sylvanas.