The difference is that Thanos was, in his eyes, doing the right thing for everyone while Sylvanas is just commiting horrible atrocities just for the sake of it.
Thanos also died twice for it while Sylvanas will just be redeemed in a very stupid way.
She intended to kill Malfurion but failed, Teldrassil was a spur-of-the-moment replacement. Or she was just triggered when Delaryn said the word hope.
She intended to decapitate the Alliance leadership at Lordaeron but Jaina easily countered it so she just kind of blew up her own city for nothing.
She intended for Derek to kill Jaina, but Baine freed him before she could finish brainwashing him.
She intended to recruit the Zandalari for their fleet but then the Alliance wrecked it before they joined.
She tried to assassinate Thrall but it failed cause she thought two random Forsaken could somehow take him on.
She intended to use Baine as bait to draw out and kill other potential rebels, but this failed because she thought a few angry Sunreavers could somehow stop Thrall, Saurfang and Jaina.
She intended to crush the Rebellion at Orgrimmar but Saurfang tricked her into admitting she doesn’t care about the Horde so she lost control and had to flee.
IIRC, the only plan of hers that actually works perfectly is teaming up with Azshara to destroy both fleets.
It would be more accurate to state that she usually messes up but never faces any consequences for her actions. She runs away screaming I’LL GET YOU NEXT TIME like a cartoon villain. No one can ever really hurt her since she’s got super thick plot armor, but at the same time, she’s such a perpetual screwup that she usually defeats herself anyway.
I would too love to kill her this expansion already, but there’s 0 chance of that happening.
Well, at the end of 8.2.5 as a loyalist you get told “Oh you know, all of this was calculated by sylvanas as one possible outcome” and then sent to Ghostlands.
She wanted death on both sides, doesn’t need the Horde now, though she did before, and is playing a larger game with gods. Her plan succeeded. I don’t like it, but that’s what it is.
Also, Baine wasn’t gonna die. Look up the loyalist story.