He’s been bad since WC3.
Proud and full of himself, he takes on too much in fighting the Scourge, makes difficult choices that lead to atrocity as a necessary evil and then pursues Mal’ganis ruthlessly out of vengeance and takes up a cursed sword as yet another step to do bad things for good.
He defeats the dreadlord and the scourge forces, then sprints off into the frozen wastes to… join the scourge? Fast forward and he returns an obvious villain decked out in skulls, set to genocide the people he gave it all to save from the undead.
That part has always been bad writing and further post hoc elaboration didn’t make it better beyond hinting at Arthas’ unbridled ego, narcissism and demanding possessiveness. These would be interesting traits were he not written as a completely different character later on, bereft of such flaws to exploit and reduced to an echo-y voiced cartoon villain.
It would have made a better story to have Arthas return from Northrend a cold eyed and hard hearted prince, scarred by his experiences with his old issues roiling underneath. Inheriting his father’s crown, he’d become even more self assured and grim, bringing that sinister sword to future battles and further atrocities. A cruel, harsh figure to balance the classical good guy Alliance image. He has been proven correct in all that he did and he will NEVER allow Lordaeron to fall!
Inflexible, proud, ruthless and probably a bit touched by his new sword, Arthas, King of Lordaeron becomes an important figure in Wrath of the Lich King as Ner’zhul calls for him. The long game seeded with the sword leads Arthas to incredible acts of brutality against friends and foes in this second Northrend war that alienates him from old allies.
As the armies of Alliance, Horde and Argent Crusade batter down the walls of Icecrown, Arthas, scarred further by the Lich King’s trap at the Wrath Gate charges ahead intent on finishing this madness and destroying Ner’zhul for all the death he inflicted on Lordaeron’s people. Arthas, dark king and hero of his now terrified nation battles the Lich King alongside Azeroth’s heroes and as the fiend finally falls, King Arthas accepts the call to take his place on the Frozen Throne in a final act of sacrifice.
The Helm of Domination is placed on his troubled, weary brow and Arthas redeems his past cruelty by becoming the Jailor of the Damned, forever to keep the Scourge at bay and protecting his people. And Queen Jaina cries.
I don’t know, that seems like an alright alternative.