@
Super Dickmann
The point is that Arthas has both a sympathetic cause (wanting to save his people from the Scourge) as well as a scapegoat to offload his culpability (Helm of Domination mind control)
Sylvanas only had a sympathetic cause in defeating the Lich King until his defeat, after which her actions have no justifiable cause for her evil deeds. More importantly though is that she does not have a scapegoat for culpability like Arthas does, given what we know we have to assume she is doing all the evil stuff like burnig down Teldrassil and raising the survivors of her own free will, which immediately makes her come off worse.
Les us compare two moments from each character to contrast:
Arthas:
Purge of Stratholme. Seen as a questionable act at best and oftne declared his darkest moment and most definitely done by hiw own free will even with Mal'ganis goading him. This act is done by his genuine desire to do good and help his people, he decides to do this evil thing because he considered the realm as a whole more important.
Unleashing the Scourge. Causes the death of potentially thousands and launches the various kingdoms into disarray. No doubt given that this is evil, all the characters acknowledge it as so. Done under the influence of the Helm of Domination, which we can now assume to at least some extent bends you to the Jailers will.
So in short Arthas had either justifiable cause, or an exoerating event that makes a redemption possible. It will still feel weird since it would be a retcon from how he was shown in most of WotLK, but even then Blizzard planted the seeds of his redemption with his guilt, and his seeming remorse when he dies.
Sylvanas:
Human experimentation. Kidnaps and experiments on live subjects to create a new plague of Undeath. Justifiable cause being that she wants to defeat the Lich King, a known and credible threat. Done entirely by her own free will despite knowledge that doing so makes her a hypocrite.
Burning of Teldrassil: Burns down Teldrassil, at that point filled with almost nothing but civilians, destroying the civilization and making its inhabitants refugees and possibly an endangered species. Done for the purpose of sparking an even more destructive war with the end goal being more casualties. As far as we known done entirely be her own free will.
Sylvanas has done her evil acts of her own free will (that we know of), and her most justifiable endgoal was revenge on the Lich King, a goal she completed, leaving her subsequent actions without justifiable cause.
The most important thing though is that there has been time since Arthas did his evil acts, leaving plenty of time for small retcons here and there to slowly acclimatize players into acepting his potential redemption. In comparision Sylvanas' evil acts are the setup for the very expansion in which we would presumably get her redemption, which would be a supremely bad writing move.