Now that Shadowlands has been released, people are forgetting about what sent Sylvanas down this dark path in the first place. Remember her decision at Icecrown to obliterate herself on saronite? And the experience she had in the Maw afterwards?
As the Arbiter was supposedly "active" back then, Sylvanas then was apparently judged to have deserved her fate. But in your honest opinion, was it a justified judgment of Sylvanas, given everything she had endured? She no longer cared about the faction war or the Horde. She only wanted peace, and oblivion. But even that was ultimately denied her. However, we should remember that she was also conducting experiments on human captives and Forsaken prisoners in the Undercity, and was not above blackmailing even her former kin in Silvermoon as was the case in Lor'themar's short story. We should also remember that she was apparently responsible for the Wrath Gate (as per a recent interview) that nearly caused the two factions to go to open war, wiping out all remaining trust between Varian and Thrall, which destroyed Saurfang's son's life, and inflicted nearly 9,000 total casualties among both factions.
Sylvanas loathes the Shadowlands. Because in her view, it is an unjust and tyrannical system which caused her suffering even though she had "no choice" in her fate being Forsaken, as she remarked to her sister Alleria in her reunion. Is her view at all justified? And either way, did she really deserve eternal torment for her crimes as of the end of the Lich King expansion?
Also, why exactly are Sylvanas and Nathanos so cruel and ruthless, comparatively? Look at Alonsus Faol, Calia Menethil or Derek Proudmoore, they all suffered considerably, dying horrible deaths against their will, yet they are not angry, bitter, or raging at the world, they are still basically the same exact people they were in life. Look at the Forsaken whom Sylvanas killed in Arathi, they all cared deeply for their old lives, for their family members, and some were willing to betray their entire faction to be with them. Look at the death knights, Thassarian, who would probably die to protect his living sister Leryssa instead of trying to secretly ambush and kill her, and his friendship with Koltira, who also seems honorable. Even look at Bolvar, who still loves his "Shining Star" Taelia, enough to honor her request to look into the Maw despite the risks of doing so, and who also seems to still care deeply for Anduin, his other "child". In short, they were all slain and raised (well, maybe not Bolvar exactly, but he has experienced most of the same things the others have). But they are not nearly as hateful or jaded as Sylvanas or Nathanos, despite the two being loyal and steadfast members of the Alliance in life just like the rest of them. Why is that?