Here's the Game of Thrones fan theory that Jon Snow will end up killing Daenerys

Martin’s vision for the story took shape in the early 90’s. The journey may have meandered, but its destination is unchanged.
It is an unfortunate truth that a story told to millions over three decades cannot conceal its own ending - the surprising ultimately becomes the predictable.
Jon, Dany, and Tyrion are the story’s heroes and Jon and Dany will join in marriage, as is foreshadowed in the House of the Undying.
The first Long Night ended through diplomacy. The Wall was created by the Others. A pact was sealed and sacrifices were given at the Nightfort.
Azor Ahai engineered this peace by thrusting his dragonsteel blade Lightbringer into the chest of his wife, Nissa Nissa, transforming her into an Other and ruling as the book!Night’s King who famously took a corpse bride.
Jon Snow will end the upcoming War for the Dawn by doing the same to his wife, Daenerys.
The two rule Westeros together as man and white walker - our promised bittersweet ending.