/waits a little more
Okay! If you're still here, I assume you've already done the quest or don't care.
Arthas is perhaps a unique character in the Warcraft lore in that he's more or less uniquely respected by the fandom. Not everyone bought into Kael'thas's turn to evil. People make jokes about Illidan being emo. But Arthas?
While he still ends up in silly comics and one of my coworker's theories on why he sits on a frozen throne is that he has hemmorhoids, there's little disagreement when it comes to his status as big villain on the totem pole. He has the kind of reputation that you can't buy. He was a defining character in Warcraft III and one of the major forces that shaped Azeroth as we know it.
But Arthas is more than just a character from Warcraft III. He was also us, the hero of the human campaign. More than many other lore characters, we may feel we know him, because we played as him (even if we thought everything he did was completely screwed up).
I admit, when I saw this quest text, I thought it may have been a sign that Arthas could eventually be redeemed. But when I met with Tirion Fordring at the Cathedral of Shadows there was no saving Arthas's heart. Arthas taunts Tirion for believing he could be redeemed and Tirion himself destroys the heart, obviously coming to the same conclusion as Arthas, that there is no turning back from being the Lich King.
I was a little saddened at the end of it, because I'd seen that glimmer of hope, but as I thought over the quest chain that led me here, I came to realize something, that there had been some measure of good still within Arthas.
While running errands for Koltira Deathweaver, the player eventually encounters a ghostly little boy called Matthias Lehner, an anagram for Arthas Menethil (thank you Wowhead). Matthias repeatedly tells the player how futile it is to fight Arthas given all his power, his lack of humanity, and also says that he can only be seen by the player because he or she touched something of Arthas's.
As a part of the Lich King it would be natural that Matthias would try to demoralize the player, but when the player first finds Arthas's heart, this little boy helps him/her hide undetected. He also warns the player that next time Arthas won't leave the heart unguarded. And why do that if he was wholly a part of the Lich King?
We did see Arthas's last bit of humanity. Though he didn't use such words, it feels like he came to say good-bye.
And now we're going to avenge him.