She’s been training as an assassin for years, building Jon up as the NK killer but having Arya do it is very GoT.
What exactly do you think she has been training for the last 4 seasons ?
I think her killing the NK fits the narrative perfectly
It's what she has been destined to do from way back in the first season eg what do we tell death ?
The NK is death
It was right in front of our eyes the whole time
First things first, the Night King doesn't exist in the books, so whoever was going to kill him wasn't going to be something decided by GRRM. The showrunners stated in Behind the Episode that they chose Arya because people wouldn't expect it... So my point was that Arya doesn't fit thematically to be the one to kill the Night King.
It's interesting that people run with small remarks like 'What do we tell death?' as some kind of evidence of her destiny yet leave out what really kick-started Arya's story; watching her father be beheaded by his enemies. Her story is not about becoming a bad-ass assassin for the sake of being a bad-ass assassin. It's about reaching a point where she can inflict revenge and justice upon the
people who have hurt her and the people she cared about:
Meryn Trant - Syrio Forel
Walder Frey - Robb & Catelyn
Joffrey/Littlefinger - Ned (beaten to Joffrey)
Okay, so there is her Faceless Men storyline, but that storyline concludes with her saying this:
Jaqen h'ghar: Finally, a girl is no one.
Arya Stark: A girl is Arya Stark of Winterfell, and I'm going home.
That storyline was about her being reminded of who she really is. She is not some faceless assassin. She is Arya Stark, and with that comes her grievances and enemies. Her enemies, listed above, is what she has trained for.
I understand there is a lot of references to the faceless God and Death during this plot. However I don't agree that Death = Dead/Night King, I think they are different concepts. Death is something that is coming for everyone and The Faceless Men/Arya is trained in dealing it out and avoiding it (looking at Death in the face and saying not today etc). This is where I think D&D just conflated the two ideas and got it wrong.
The tl;dr is that Arya's arc is about exacting revenge on those who hurt her. Not being the hero. D&D basically confirmed they just made this up; so in my opinion they don't understand her character.
So, who instead?
1)
Jon Snow. Yeah, it may have been painfully obvious, but including twists for the sake of twists and to shock the audience is not good writing. His entire story and arc lead to this moment. So what purpose does he serve now? Why would he give a flying f**k who sits on the Iron Throne? I guess you could argue because he loves Dany, but if they reduce Jon Snow from hero to loved up pawn then put Jon Snow down as another character they don't understand.
2)
Jaime Lannister. The Kingslayer. Why? Well, what was the point of him riding North if not to be a hero? Jon etc sacrificed a dragon and men to gain Jaime Lannister in Season 7 (that's all he got from taking a wight to King's Landing) and he essentially did nothing. Jaime's redemption arc is well-documented, as his ongoing battle with honour and doing the right thing. There are the empty pages in his Kingsguard entry regarding his heroic deeds. Him killing the Night King would have been the perfect completion of his redemption story and would have transformed Kingslayer, an insult he received prior to redemption, to something heroic post-redemption arc.
Instead, we got Arya killing him, because Arya is cool and a bad-ass and is cool and I guess the Night King was on her list or something. Ugh.