The Game of Thrones Season 8 premiere is finally here (along with winter) and among the prophecies yet to come to pass is the one about Jaime killing Cersei on Game of Thrones. It's been a long-held fan theory that he will, especially since the books contain a more explicit prophecy saying the Kingslayer will become a Queenslayer. But let's look at the facts.
Cersei currently sits upon the Iron Throne, and while other Westerosi rulers like Jon Snow and Daenerys Targaryen are busy putting their differences aside to fight an army of ice zombies, Cersei is... not! The paranoid queen is descending into a bit of madness herself, displaying creepy parallels with the Mad King Aerys. (Then again, so is Dany, but I digress.) Cersei knows winter is here and the White Walkers are heading south. But instead of banding together for all humanity, she's still preoccupied with maintaining her control of Westeros and defending the Iron Throne against usurpers. Like, very much missing the the forest for the trees, there, Cers!
Jaime has already murdered a king he'd sworn fealty to in order to protect the realm. With Cersei exhibiting similar behaviors to Aerys, it doesn't seem like a huge logical leap for Jaime to kill her, too. Season 7 showed him growing increasingly disturbed by her actions, so it definitely seems like he's grappling with something.
Then, there's that prophecy. This is where the books and the series diverge a little bit. In the books, a flashback to Cersei's childhood shows her visiting a witch named Magy the Frog who divines her future. It's the scene that opens Season 5 of the show, right before Cersei heads off to her father's funeral. In both the book and the series, Cersei learns that she will, indeed, be queen "for a time...until there comes another, younger and more beautiful, to cast you down and take all that you hold dear."
But the books included a line which the series cut. In prophesying Cersei's future, Magy concludes, "And when your tears have drowned you, the valonqar shall wrap his hands about your pale white throat and choke the life from you." Valonqar is high Valyrian for "little brother." Cersei reasonably assumes this refers to Tyrion, her despised little brother. However, Jaime was the second-born twin, which technically also makes him her little brother. It would be a powerfully symbolic and poetic conclusion to the story for Jaime to have to murder his twin sister, lover, and the mother of his children to save humanity.
Plus, the strangulation imagery evokes Jaime's prosthetic golden hand, the one he was fitted with after his own hand was cut off. Cersei famously despised it, because it served as a reminder that Jaime was no longer the greatest swordsman in the kingdom. It would be bitterly poetic justice for Jaime to take her out with that very hand.
As you can see, the case for Jaime murdering Cersei is strong. But that final line of the prophecy was left out of the series, so it's always possible showrunners David Benioff and D. B. Weiss have something else up their sleeves.