Game of Thrones Season 7 has been shockingly low on grisly deaths, outside of a few secondary characters like Lady Olenna, Thoros of Myr, and, of course, one now-zombified Viserion the dragon. So, in the Season 7 finale, the show might make up for the quieter, shorter season with some more eventful moments. So, will Sansa & Arya kill Littlefinger on Game of Thrones? After "Eastwatch," it might be time for the show to finally deliver a payoff for the constant arguing the sisters have been doing since Arya returned to the North. Arya certainly has had plenty of real bitterness towards her sister. Sansa was always the well-mannered daughter, with no desire to learn about traditionally male pursuits like sword fighting, the way Arya did — and Sansa did remain in King's Landing after Arya escaped, living an outwardly more pleasant life than Arya's journey into the woods. But the reality is that both sisters have suffered and fans hope Littlefinger doesn't drive them apart. Maybe uniting to kill them will give them common ground.
Bran already revealed that he knows at least one detail of Littlefinger's King's Landing plotting, when he referenced Baelish's famous catchphrase "chaos is a ladder," while holding the dagger that Littlefinger had planted on Bran's assassin in hopes of sparking what eventually became the War of Five Kings. If Bran knows about "chaos," he might also know that Littlefinger betrayed Ned Stark and ultimately aided in his execution, helped frame Sansa for Joffrey's death, and even traded Sansa to Ramsay Bolton, where she was tortured (frankly, Sansa should have never trusted Baelish after this either).
In addition to Bran's power, Arya is a stealth expert, yet she found the planted note Littlefinger wanted to lead her to, and was so bad at stealing it that he overheard their entire conversation? An enterprising Reddit user theorizes that Arya was manipulating Littlefinger throughout the whole fight she had with Sansa. "Arya was trained by assassins. She is far too stealthy to let [Littlefinger] know that he is being followed, unless she did this deliberately. In S7E4, Arya walks onto Brienne and Pod sparring just as Brienne says, 'Don’t go where your enemy leads you.'" There are wilder guesses (like "Arya has already stolen Littlefinger's face") but this one seems plausible.
It's a compelling theory, especially when examining that speech Arya uses to threaten Sansa, which, on second look, doesn't really make any sense. She criticizes Sansa for sending the note that was clearly written with Cersei's influence, even though Robb easily saw through it. She claims that she would "never serve a Lannister" even though she literally was Tywin Lannister's servant. She even taunts Sansa with the possibility of wanting to steal her face, even though Arya has never wanted to be a "lady," but a powerful warrior.
So it seems very possible that Littlefinger's plan was unsuccessful, and the sisters are already in cahoots. But if they do kill Littlefinger, there could be some serious ramifications. Right now, Littlefinger is in Winterfell as the representative of House Arryn, which rules over the Vale, one of the Seven Kingdoms. Before Littlefinger rode into battle against the Boltons, Jon Snow was losing badly, and their defection from Queen Cersei is a major negotiating point for Jon and Daenerys to use in their meeting. Killing Littlefinger could jeopardize that alliance.
However, it's been established in past seasons that the other Knights of the Vale, like Bronze Yohn Royce, who appeared in a Season 7 episode, don't approve of Littlefinger controlling their house, which is technically ruled by Lord Robin Arryn, the Stark sisters' cousin (a scene in an earlier season showed Baelish bribing Robin with gifts). While it would certainly be a bold move to kill or execute Littlefinger for betraying Ned, it might not mean the loss of the Vale as an ally. Of course, it could also be preserved by Arya stealing Littlefinger's face, like the above Reddit user theorized, and taking his place.
It's hard to accept that Arya and Sansa Stark were foolish enough to fall for Littlefinger's attempt at manipulation. And if you accept that they're smart enough to be on the same wavelength, they could instead be dialing up their natural differences into a feud in order to lure Littlefinger to his own death. And, if Bran provides the evidence, it seems almost inevitable that Littlefinger will die by the end of Game of Thrones Season 7.