It has been a long time since the true death of Arthas Menethil. The Icecrown Citadel raid in which we faced him was released late in 2009, and I believe that gating kept players from actually getting to him until 2010.
Arthas was basically the Darth Vader of Warcraft. He began with a great deal of promise - the heir to the beloved King Terenas and a Paladin training under Uther the Lightbringer himself. A childhood friend of Varian's - a surrogate brother, really, whose family took in the boy who was far too young to be a king in exile - and the man who, probably, would have been Jaina Proudmoore's husband eventually, Arthas "should" have been one of Azeroth's most beloved heroes.
But his fate was the opposite. After taking up Frostmourne, Arthas walked a path that made him the most infamous human in the entire Warcraft universe, and one of the deadliest villains Azeroth would ever face. His rule of the Scourge was not just potentially world-ending, but also cruel, twisting other former heroes like himself into monstrous butchers who turned on their own friends and family.
But how much of it was really his fault?
Our exposure to Arthas before he became the Lich King takes place within Warcraft III - first the original game, Reign of Chaos, and then the expansion, the Frozen Throne. When we meet Arthas, he seems mostly benign - a little cocky, but pretty dedicated to serving the Light and keeping his people safe.
But thanks to Kel'thuzad, the horrors of the Plague of Undeath are unleashed on Lordaeron, and while Arthas and Jaina quickly deal with the necromancer, it is not enough to stop the onslaught. Arthas comes to the city of Stratholme, and this is really the big turning point for him. Seeing that the infected grain has made it to the city, Arthas decides that they must put down the citizens of the city before they arise as undead enemies.
Jaina leaves him, and Arthas perhaps arrogantly disbands the Knights of the Silver Hand when Uther refuses to assist him in the cull. Arthas goes ahead with it, meeting the Dreadlord Mal'ganis, who taunts him into traveling to Northrend.
Here, Arthas' obsession becomes more troubling. In particular, when he is called back by his father, he has a group of mercenaries burn his ships, and then blames the act on them, using his army to silence them before they can protest. Finally, in his last confrontation with Mal'ganis, he finds the runesword Frostmourne and, against the dire warnings of his friend Muradin Bronzebeard, he takes the sword, accepting the curse it bears in order to defeat the demon.
After this point, though, to what extent can we blame Arthas for his actions? Frostmourne is a trap - created by the Lich King in order to anoint someone as his champion. While it does, yes, allow Arthas to defeat Mal'ganis, the true effect is that it frees the Lich King from the Burning Legion's control. Very abruptly, Arthas goes from fighting the Scourge to being its most important leader. But because the sword has stolen his very soul, the Arthas we see is kind of just a shell.
"Soul" is one of those concepts that we all kind of understand, but have different conceptions of it. In a fantasy world like Warcraft, one's soul has more clear and studied properties. It seems that in Warcraft, the Soul is kind of the substance of a person's free will. I don't know if it is one's consciousness as well (which is my general interpretation of the word,) but the most important idea here is that Arthas loses some fundamental part of his being when his soul is taken by Frostmourne.
Yet he doesn't seem to be a total puppet. The Lich King uses Arthas' natural ingenuity and egoism to make him into a fearsome death knight. One interpretation of Frostmourne's effect is that it stole the good half of Arthas' personality. His conscience is utterly gone (something he even remarks upon to Kel'thuzad's ghost as the undead campaign begins in WCIII,) but he still seems to think and feel the way he did before his transformation in all other respects.
So really, the question is this: After getting Frostmourne, is Arthas truly Arthas? Or is he just an animated corpse that has the intelligence and knowledge of the person who once lived within that skin?
Things get far more complicated, of course, after the Frozen Throne campaign. Arthas is recalled to Icecrown to stop Illidan's assault on the Throne. After his victory, he shatters the Throne to retrieve the armor to which the Lich King is bound. Placing the Crown of Domination on his head, Arthas becomes the new Lich King.
But this fusion of personalities - the incorporeal presence of Ner'zhul with the fully-functional body of Arthas - has always been kind of a mystery. In the Arthas novel, it's suggested that shortly after this fusion, Arthas' personality overwhelms and consumes that of Ner'zhul - if the old Orc Shaman exists in any form afterwards, he could only be a helpless observer. This is further complicated by the idea that the Lich King might actually be a separate entity from both Arthas and Ner'zhul, which now exists in conjunction with Bolvar. If that's the case, we really don't have any clue to where this unaffiliated Lich King entity came from. Perhaps it was from Kil'jaeden's demonic magic that birthed him, yet the Pure Lich King seems to have nothing but disdain for the Legion.
When Frostmourne is shattered, the thousands of souls taken by it - most importantly that of King Terenas - escape, and they subdue Arthas to allow the freshly-resurrected heroes (us) to deliver the killing blow. The helmet falls from him and we hear Arthas' last words spoken to the ghost of his father - that he sees only darkness before him.
A grim fate to be certain. Later, while Sylvanas drifts through the void of death, she seems to see Arthas there (before she is resurrected by the Val'kyr.) It may indeed be the Void - the dark dimension that Ner'zhul drew his power from, and that exists outside even the Twisting Nether.
Arthas' crimes were many, but if it wasn't really him committing them after taking up Frostmourne, does he deserve this fate? Certainly the burning of the ships and the betrayal of the mercenaries were both pretty horrible acts, but an eternity adrift in nothingness? That seems too harsh. The genocidal march through Quel'Thalas and the ravaging of his own Lordaeron were acts committed after the very thing that should bear punishment - his soul - was not longer in control of his actions.
Arthas should perhaps be forgiven for these crimes for the same reason we make exceptions in the real world for people who commit crimes because of mental illness. They literally don't have control of their bodies, or they are operating under convincing hallucinations that affect their judgment. Arthas wasn't at the wheel, as it were, when he was slaughtering people throughout Lordaeron.
On top of all this, there's another complication, but it's one that I don't want to get into until we have a spoiler break. Arthas wasn't just driven mad like Deathwing, and he didn't undergo some kind of nihilist epiphany like Sargeras. Arthas had his self-control wrenched away from him. Yes, he took up Frostmourne voluntarily, but he did so out of a misguided attempt to protect his own people. Arthas was the Lich King's victim, perhaps more than he was the Lich King himself.
In addition... (Legion Spoilers)
Frost Death Knights will be acquiring a pair of swords made from the shards of Frostmourne in Legion. They will arrive at Icecrown Citadel, where The Lich King (Bolvar Fordragon) welcomes them and tells them to help by putting down some maddened spirits who escaped from the blade but have not made the transition into a more normal afterlife so smoothly.
After doing so and collecting the shards from which the souls had poured out, Bolvar summons you to the Frozen Throne and constructs the Blades of the Fallen Prince out of the shards. Before you can use them, however, you need to help subdue the two souls that have not left them.
You enter the blades only to discover that those two souls are Ner'zhul and Arthas. And the big shock is that not only is Ner'zhul still around as a spirit, but he's also the one who seems to be in control. Ner'zhul sends Arthas against you while attacking with various frost attacks. Arthas seems to be pleading to be released from Ner'zhul's control.
This really calls into question a lot about what we've seen in the past. Since Arthas dawned the helmet, the impression we've been given (and told, explicitly in the novel,) is that Arthas is the dominant personality. After his death, Sylvanas believed she had seen Arthas' soul drifting in the Void.
So how does this all work? Is this a massive continuity error? Or has Ner'zhul been cleverly manipulating what we've been able to see? Or is the Lich King - as an independent entity distinct from all the people who have born that title - really the thing in control, and the "Lich Kings" have merely been puppets just like anyone else raised by the Scourge?
Setting aside his soul, was Arthas' intelligence even in control of the Lich King? In Howling Fjord, the Lich King mentions having been a shaman once if he catches you in the spirit world. Was this Arthas drawing upon Ner'zhul's past knowledge? Or was that the big hint that Ner'zhul was still calling the shots?
And if Arthas' soul - his true, essential self - has been in Frostmourne all this time, even after it was shattered by the Ashbringer, then it seems more convincing than ever that he can't be held responsible for his acts as a Death Knight or as the Lich King.
It's been almost six years since Arthas' demise, and while on a meta-level it's been sad to lose such an iconic villain, in-universe we've all been relieved that he's gone. But maybe this fate was far too cruel for him. Maybe he's due for some redemption.