Warning: spoilers for Game of Thrones below!
Leading up to the season seven finale of Game of Thrones, plenty of theories popped up about the eventual fate of Littlefinger: would Arya murder him? Should Sansa kill him instead? Would Arya take out Sansa because of her loyalty to the former Master of Coin? Although all would have been interesting developments, "The Dragon and the Wolf" finally put an end to all of our questions. Littlefinger, after spending years manipulating the Starks and essentially igniting the War of the Five Kings, was sentenced to death by Sansa at Winterfell.
Despite the appearance of tension between Arya and Sansa over the past few episodes, the sisters had actually been working together all along (all the credit in the world to Arya, because she laid her "I hate my sister" routine on THICK). After tricking Littlefinger into thinking that he's attending a trial for Arya's past misdeeds in the Great Hall, Sansa surprises the hell out of him by actually announcing that it's his life that's at risk. Unfortunately for Littlefinger, Sansa's mind is completely made up. She orders Arya to slit Littlefinger's throat with the very Valyrian steel dagger he uses in season one to start drama between the Starks and the Lannisters, but there's another reason why his death is so satisfying.
In season one, Littlefinger figuratively stabs Ned in the back by literally holding the dagger to his throat and chiding the Stark patriarch for trusting him to begin with. While Sansa gives the order for Littlefinger's death, it's Arya who actually does the deed, which is fitting if you take into account just how much Arya has started to look just like her father lately. It's as if Ned himself gets revenge on Littlefinger for destroying their family, slicing open his throat the same way Littlefinger almost did years earlier.
Like Sansa says, "There's no justice in the world, not unless we make it."