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Super Dickmann
I am not arguing that it would be good writing, but rather that on a meta level Arthas' redemption is more plausible because of both the exenuating circumstances the writers have piled onto him since WotLK, but also the simple fact that we are sufficiently far removed from his actions that we can argue that his redemption falls into the realm of retcon, rather than blatant narrative suicide.
The point about Kerrigan is valid, but her redemption was in a sense perfectly valid within the confines of its own trilogy (or rather first 2 since LotV barely mentioned her). It was a bad retcon when seen as a whole. but the writers did take steps to ease the transition from Starcraft Kerrigan to Starcraft 2 Kerrigan.
And yes, the arguments are probably not entirely fair, but my point there is more that within the story right now Arthas has grounds for aredemption, whereas Sylvanas does not, regardless of whether her actions are objectively less damaging. (At least up until she broke the Helm, hard to argue death of a continent to death of a universe)