That’s the entire point, that’s what I meant. He was a subversion, you can’t subvert without setting up the dominoes. Prince Charming (aka Great Value Arthas) gets a pretty heroic looking introduction at the start of Shrek 2, and it’s only after he arrives in Far Far Away that it’s subverted. Up until then the implication is that he’s a somewhat goofy but heroic guy who was ready to slay a dragon and save the princess.
Arthas is the same, the manual and even his introduction are of a character who is classically heroic, right down to the looks and voice. But when he has to make a hard decision that involves a very difficult choice, he chooses to wipe out everyone, including those who very likely could have been saved, and it doesn’t even do anything. And then he just keeps getting worse and worse, all before getting Frostmourne. And then after Frostmourne, and then becoming the Lich King, which per the Arthas novel was by Wrath 100% ol’ Evil Arty, he proceeded to inflict, again, the worst fate that can befall a sentient being.
Discounting Shadowlands, because let’s face it the sheer scale of what they were trying to get across is… dumb, and cannot be related to at all, Sylvanas in terms of ‘bad stuff done’ is simply not on par with what Arthas did, and that was after what was inflicted on her. Arthas wiped out an entire city and killed the people who followed him before he so much as got his mitts on the It’s Not A Phase Mom sword, Sylvanas had to have her body, mind, and soul violated to the nth degree before she did any Scourge things, and broke free the moment she was able to.
In the context of Shadowlands, would Arthas have ended up in Revendreth with the chance for redemption? Given what we’ve seen, sure, right along with Sylvanas. But honestly post BfA it’s almost impossible to reconcile the writing of the past with more recent years. It’s incredibly difficult to compare the two because Arthas was killed when the writing was… well, very different and the scale was far lower.
If we factor in the mournblades and whatnot, and say they’re not responsible for their actions from the moment they either get, or are killed by, Frostmourne, then Arthas’ actions still weigh far more heavily against him than Sylvanas.
The real dude who should have been tossed into the Maw immediately, that said, is Kel’thuzzad.