Arthas has a much larger body count than Sylvanas and his Scourge made a giant out of dead women and children that were all fully aware as they screamed for you to put them out of their misery. He has done absolutely everything Sylvanas has done + another one in pretty much every aspect and Scholomance alone, which is only one outpost of one aspect of the Scourge dwarfs the entire activity of the R.A.S. over their seventeen year in-game existence. There's much that can be said about Sylvanas, but let's not kid ourselves here.
Further, the whole gist of Sylvanas's argument and why this motivation would halfway work if it was her own plan rather than some BS Satan told her is that her actions are reversible. Sure, those souls are now burning in hell and sure, it sucks to be them, but once she has the necessary power she'll free them and everyone else into paradise. The complete sociopathy of this kind of thinking where she judges herself as being able to decide that women and children are fine being picked apart by Sauron wannabes plus the iffiness of everyone getting a happily ever after no matter what harm they did in life would already be reason enough for her to be a main antagonist, while also grounding her perspective. The game toys with this perfectly servicable animu tier-goal, but it doesn't go the whole distance because we know that she has no independent plan to actually do this, but is entirely reliant on someone who obviously doesn't give two shits about this. This story is reliant on Sylvanas, an extremely vain and prideful character who's based most of her decisions on what helps her and thinking she knows best, to be both inexplicably trusting and inexplicably deferring to a figure she has no reason to trust. Nevermind what Kel'thuzad did to Lordaeron you can at least say she's past giving a shit about that, what he brought on her alone to get her into this point alone while not being essential to giving her more power like the Jailer should be reason alone for her to conspire to get rid of him. Her not caring about Kel'thuzad because she's already done so much bad shit and is desensitized would be one thing, her not caring about Kel'thuzad but trusting the Jailer and feeling bad about Anduin is quite another.
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Minikin
Arthas and Anduin's situations are similar only in terms of visual design. Past that Anduin's basically a sockpuppet whereas Arthas was still lugging his own soul around in Frostmourne and was obviously able to think and act given we have a whole book from his perspective that largely takes place after the fact. Not to say that this discounts the Arthas redemption route, quite the contrary. Their ability to completely miss the point with Sylvanas reinforces that they'll also completely miss the point with Arthas.