This post contains spoilers from the Game Of Thrones Season 7 finale. Sansa Stark was not having a good episode in the Game of Thrones finale. Finding out via raven from Brienne that Jon Snow had bent the knee to Daenerys was bad enough. How *was* she going to explain that to the Northern Lords? Suddenly not going to King's Landing seemed the wrong move. But then Littlefinger started playing her rage like a fiddle. Pull a Cersei... Let them be killed by the Night King... You won't be next.... Until she'd had enough. Finally, Sansa destroys Littlefinger.
The show certainly played it close to the chest. It seemed like Littlefinger was winning. He was explaining Faceless Men to her. Sansa has been her own mini-Cersei for most of the season, to the point of even wearing the same hairstyle, and admitting she admired the Mad Queen for always getting what she wants.
But Sansa is a Stark, and though her teachers in life were Lannisters, Baelishes, and Boltons, that means something -- honor, duty, etc. When push came to shove and Sansa had to decide if she would kill Arya and take her place as Queen In The North over Jon Snow, she instead went to someone she trusted, who would speak the truth.
He'd seen the truth of her wedding night. He'd seen the truth of Arya's list. He would see the truth of Littlefinger for her.
Armed with this knowledge, Sansa gathered the Stark family together and took down the snake in their midst. Lord Baelish killed Lysa Arryn, Sansa was the witness. He conspired with Cersei and Joffrey to have Ned taken down. He started the entire war with the Lannisters and the Starks. The dagger was always his; he had never lost it to Tyrion.*
(*This is a huge change from the books, btw, where Littlefinger really did lose it to Tyrion, and Joffrey stole it and tried to have Bran killed in a misguided attempt to prove himself to Robert whom he still believed to be his father at that point. But it's a smart streamlining of what is otherwise a complicated plot line, and one to pin more guilt on Littlefinger.)
As Bran says: "You held a knife to his throat. You said, I did warn you not to trust me."
I don't personally know what was more satisfying: the look on Littlefinger's face when the hammer dropped, or the look on it again when Arya sliced his throat so fast, he didn't see it coming.
And that was the end of Littlefinger.
When the snows fall and the white winds blow, the lone wolf dies, but the pack survives.
The Starks will survive. Summer is a time for squabbles. In winter, we must stick together, protect one another. There has always been a Stark in Winterfell, ever since the first Long Night. Gods willing, old and new, this will stay true.