And I don’t think anyone is disputing the first part, really.
As for the second one, it’s not so easy.
Arthas pre-Frostmourne is no monster or even a bad person, that is something the book makes very clear. He is, however, a flawed person and he knows it.
He isn’t a born fighter like Varian, his faith in the light isn’t strong like Uther’s- hence the light hesitating to bless him on the day when he becomes a paladin.
That seemingly unwavering self-confidence he displays is only a facade, in truth he is deeply insecure, crushed by the expectations placed on him and haunted by the fear that he might not be good enough. That he might fail everyone. His father, Jaina, his kingdom.
And those fears only intensify with Invincible’s death. That’s the crucial moment when he swears to himself that he will never fail anyone again, that he will do anything to protect his people, no matter the cost.
And that’s exactly what he does in Stratholme. He hates every second of it btw the book is also very clear on that. What he does is still terrible and he feels terrible about it, which is why from that point on he is so obsessed with his vendetta that he becomes immune to reason and good advice. If he cannot stop this, if he cannot kill Mal’Ganis, if he cannot save his kingdom, it was all for naught.
But let’s not pretend that killing innocent people-even if it’s a necessity and you felt that there was no other choice- won’t take a toll on you and leave you broken or jaded.
Unfortunately, the warcraft universe has no therapists that will help you with your PTSD. And Arthas in Northrend clearly isn’t the same person anymore.
His people notice that he doesn’t seem to feel the cold. The light is more and more reluctant to answer his call.
And then Frostmourne presents itself as the solution to all of his problems.
Now that he’s gone so far how could he let something so trivial as a curse stop him?
Was it the right choice? Of course not, we all know that.
But if he had succeeded? We see the exuberant welcome Lordearon gives him upon his return. He reached his goal but he paid a terrible price for it- his very soul ( He didn’t know it at the time he picked up the sword but I’d argue that he would have done it nonetheless. )
TLDR:
tbh I don’t know why liking Arthas or Sylvanas as a character must be mutually exclusive. But I think people like Arthas’ story because it was good and actually made sense, and sadly you can’t say the same for Sylvanas as of bfa.