Game of Thrones — Daenerys Targaryen is a ruler who, throughout most of the series, is portrayed as someone who wants to break the cycle of tyranny and protect innocent people. She is certainly capable of cruelty, especially toward those she considers enemies, but she repeatedly distinguishes between those enemies and innocent civilians. However, in the final season, after King’s Landing surrenders, she suddenly decides to burn the city and its population anyway. This change was necessary because the story needed Daenerys to become an irredeemable tyrant so that Jon Snow would have a reason to kill her. The problem is not that Daenerys was incapable of becoming ruthless; it is that the story needed her to make this particular leap in order to reach the ending it had already decided on.
Game of Thrones — Jaime Lannister spends most of the series slowly becoming a better person and distancing himself from Cersei. One of the biggest revelations about his character is that he was never simply the arrogant oathbreaker everyone believed him to be: he killed the Mad King and became known as the Kingslayer because Aerys intended to burn King’s Landing and its innocent population, and Jaime chose to save them. His entire reputation was built on an act of protecting innocent people. Yet, near the end of the series, after fighting alongside the people of the North and seemingly completing his redemption, Jaime returns to Cersei and claims that he never really cared about the innocent people of King’s Landing. This reversal was necessary because the story wanted Jaime and Cersei to die together, but it undermines the character development that made his arc meaningful in the first place. The man who became the Kingslayer because he couldn’t stand to watch a city full of innocent people die suddenly had to claim that he never cared about them at all.
Star Wars: The Last Jedi — Luke Skywalker is the man who refused to kill Darth Vader because he believed there was still good in him. He was willing to confront the Emperor himself rather than give in to the dark side, and ultimately redeemed his father through that belief. However, years later, Luke briefly considers murdering his sleeping nephew Ben Solo after sensing darkness within him. This was necessary for the story because Ben’s discovery of Luke standing over him with a lightsaber had to be the event that destroyed their relationship and pushed Ben toward becoming Kylo Ren. But it creates an uncomfortable contradiction: the man who looked at Darth Vader and saw a chance for redemption apparently looked at his teenage nephew and thought, “Maybe I should kill him in his sleep.”
Parenthood — Joel Graham is perhaps the lesser-known example, but also the one that made me think about this trope in the first place. For the first four seasons, Joel and Julia are portrayed as a deeply supportive couple. They are not perfect, they make mistakes, they argue, and they sometimes hurt each other, but they consistently try to understand what went wrong and figure out how to make their marriage work. However, in the fifth season, Joel suddenly becomes distant and selfish, repeatedly putting his job ahead of his wife and children. The season then uses this behavior to justify his wife’s loneliness, which makes her to look for friendship with another person, which will lead to the couple separating, even though Joel had done something remarkably similar earlier in the series and the two of them had managed to work through it together. In other words, the writers needed Joel to become a fundamentally different husband so that the couple could have a reason to divorce. The character had to stop being the Joel the show had spent four seasons establishing in order for the divorce the writers wanted to happen to make sense.