Remember where Arya and Sansa are coming from
"Beyond the Wall" delves into to the past to give us a clearer idea of how a key relationship might play out after the Long Night is over. Early in the episode, Arya tells a story about sneaking into the Winterfell courtyard for a unladylike round of archery -- which her father witnessed and applauded. Years after his death, Ned continues to be a driving force for Arya. He was the model knight she pined to become. He was pure, and was punished for it. Now with the discovery of the note, his legacy is also a wedge between Arya and her sister.
But Sansa is a warrior, too. Not only does she remind Arya (and us) that she's the reason Jon Snow won the Battle of the Bastards, calling upon Littlefinger and the Knights of the Vale in the nick of time, but she's suffered "things you could never imagine." Arya knows the world doesn't let girls be what they want to be, but she also doesn't know much about what the world does want girls to be -- she escaped the system. Sansa felt the hand of men in the worst ways imaginable, from her time with Joffrey to Littlefinger's slimy manipulations to a literal violation: her rape by Ramsay Bolton. Arya can blame Sansa for not doing anything about Ned -- it's not like Arya did anything either -- but the blame is knocking a victim who's been knocked time and time again.
This seems to impact Arya, although she's on the offensive when the two collide again toward the end of the episode. After shaming her in the spot where Ned Stark used to watch his children train, then batting her around with Braavosian mind games, Arya handed a spooked Sansa the Catspaw Dagger, an item most show sleuths believe to be a Targaryen relic from the times of Aegon the Conqueror.
The question is the perpetual one for Thrones: Why? Seconds before turning over the weapon, Arya reminds her sister that, while they're on totally different paths in life, they've faced similar troubles:
We both wanted to be other people when we were younger. You wanted to be a queen sitting next to a handsome young king on the iron throne. I wanted to be a knight. To pick up a sword and go to battle. Neither of us got to be that other person.
That's why Arya keeps a bag packed with faces. In Season 6, the young Stark girl completed her training under her Faceless mentor Jaqen H'ghar by murdering the Waif. But she bungled the first assignment, sparing the life of an actress who reminded her why identity isn't all that bad. So she ditched the assassin's guild, but kept their prized possessions, the treated skin masks that make the Mission: Impossible crew seethe with envy. That is her life, she tells Sansa: She's not a knight, but may be a knight if she finds the right face and wants to slip into that life for a second or two.
Their mano-a-mano scene tenses up as Arya closes in on Sansa, turning her words, and the blade, toward a potential next victim. This could be revenge. "I can even become you," Arya says. "I wonder what it would feel like to wear those dresses."
We know Arya opts not to slice her sister's face off, but the intentions of this speech, and the reasoning behind gifting the Catspaw Dagger to Sansa, remain mysterious. Was this a change of heart? A coded call to action? Or perhaps a theatrical nudge to get Sansa playing a more brutal version of "the game"? Keep this in mind, first.