Arya was built up and addressed.
No she wasn't. D&D literally told us they did it to counter expectations, meaning they wanted it to be a surprise. D&D figured out Arya was going to kill the NK three years ago. D&D have known the ending longer than that. There isn't likely a NK in the books. Meaning, D&D probably decided it was Arya to do it. Arya hadn't even heard of the Night King until recently.
Jon's entire story arch had been fighting wights, uniting and preparing people for the Others. Jon fought the Night King. Twice. Was Jon involved with the fight with the NK when he was killed? No. I don't have a problem with Arya doing it, I have a problem with how they did it. I even made a loose prediction of Arya killing the NK while Jon is fighting him, like Eowyn and Mary.
She trained to be the best assassin for multiple seasons.
So that gives her ability to be completely invisible to the wights that she has to pass on the way to the Night King? I mean, she had to put quite an effort to keep hidden inside Winterfell, why was it so easy to sneak past wights in the open?
She was give that dagger under the weirwood tree where she assassinated TNK.
Means absolutely nothing. Bran is usually by the weirwood tree. Bran asked Jaime "How do you know there is an afterwards?" meaning Bran wasn't even sure how it ended.
Melisandre literally stares at her for 10 seconds upon entering the castle, she does not do that for anyone else.
Cool. Were we supposed to decrypt what a look means when we first watched it?
After the hound and Beric save her, two people who have been wrapped up in the Lord of Fire plot line forever, get her to Melisandre who also has been wrapped up in the Lord of Fire plot line forever, and tells her she must go do something
Aragorn was the Prince that was promised.