Sansa Stark is by far the character who has evolved the most over the course of Game of Thrones' six seasons and she became the real MVP during Season 6, Episode 9, "The Battle of the Bastards." While the HBO series was stellar throughout Season 6, Sansa smiling when Ramsay was killed on Game of Thrones was a defining moment not only for the oldest Stark daughter, but for the series in general. Considering how much the show improved in its treatment of women from Season 5 to Season 6 — particularly with Sansa's character — it's worth arguing that Sansa's smirk after orchestrating Ramsay's death on Game of Thrones was the most meaningful scene of the series in all of 2016.
Season 6 had plenty of epic moments, from Jon Snow coming back to life to the true meaning of Hodor's name being revealed. Yet, watching Sansa exact her revenge against the man who had repeatedly raped her was a rare victory in a show where many beloved characters don't get a lot of wins. In regards to Sansa's story, Season 5 of Game of Thrones was highly controversial since in George R. R. Martin's books, she did not marry — and thus, was not raped by — Ramsay Bolton. Yet, while Sansa's rape can be considered a low point of the series (and it was by many people), Sansa being responsible for Ramsay's death — and being content with that action — was a high point.
As for how her character was developed in Season 6, Sansa was given time to show what it was like to live as a victim of sexual assault after escaping Ramsay's horrifying clutches, which was a new phenomenon for Game of Thrones. She made sure that Littlefinger understood completely the torture she lived through by him marrying her off to Ramsay — making him acknowledge the grotesque consequences of his actions. She also told Jon that he did not understand the full extent of what Ramsay was capable of like she did since she had been forced to live in fear for her life as his wife.
All of this led to the big battle in the penultimate episode of Season 6 between Jon Snow and Ramsay Bolton. Jon only won due to his sister's masterful planning and while he had the opportunity to kill Ramsay, Jon — and the show — made the right choice in having Sansa be the one to deal the deathblow. She gave a brief, but powerful speech about how Ramsay would not be remembered in history. Then, befitting his sadistic nature, she unleashed his own hounds on him — and they ate him alive. Sansa, who started Game of Thrones as a girl who could only dream of becoming a princess, watched the gruesome scene long enough to confirm that Ramsay would in fact die. Then, as she left him to be mauled, the slightest of smiles emerged on her face. It was a smirk for the ages and props to Sophie Turner for a masterful and subtle performance.
While there are many ways you could interpret the smirk, I am not ready to say that it indicated Sansa is turning into a completely ruthless killer like her sister Arya, although it surely showed Sansa's dark side as Turner noted to Vulture. For me, in that moment, it highlighted that she now knows how to play the game of thrones in order to survive — and better yet, thrive. She is more than aware that she's entitled and deserves to rule Winterfell and kill the man who tortured her. As a character who so often was not allowed to have any real power, who really has just been getting by since her father was killed in Season 1, and who lived through relationships with the two worst characters of the entire series (Joffrey and Ramsay), murdering Ramsay was her moment to show how strong and merciless she could be to a person who truly deserved her wrath.
Sean T. Collins of Vulture wrote that the characters on Game of Thrones frequently pay for their acts of vengeance and so fans should feel uneasy about Sansa's satisfied smirk. While I agree that revenge is rarely the answer in real life or on Game of Thrones, something felt different about Sansa's revenge. It was totally a move out of Daenerys' book — like when she had set flame to the Dothraki temple earlier in Season 6. These power acts by Sansa and Daenerys were examples of women giving the men who had tried to destroy them a taste of their own medicine and ending up victorious. Sansa has seen firsthand the horrors of Game of Thrones and she is not going to be the victim anymore.
Perhaps I'm being naively optimistic, but I don't think Ramsay's death will negatively impact Sansa in the larger political sense since he was never the rightful ruler of the North in the first place. Nor will it negatively impact her emotional well-being. Instead, with the last of the Boltons defeated, I felt a sense of closure for this chapter of her life. The memory of being raped will most likely stick with her forever, but that smile indicated that she has the ability to move on with her life now with Ramsay gone. He might have claimed that he'll always be with her — that he's a part of her — but I believe Sansa will never be defined by what Ramsay Bolton did to her. And it's all thanks to a supremely well-acted smile in the most meaningful moment of Game of Thrones Season 6.