There is one constant in Westeros. One rule. Ned Stark learned it; his son and his wife learned it; Stannis Baratheon and Oberyn Martell learned it. The rule is there are no rules, and if you're living your life under a strict ideology, set of rules, beliefs, or goals, you won't last very long. The key to survival in Game of Thrones is flexibility and adaptiveness, which could be bad news for one of the show's best, most complicated characters Jaime Lannister.
In what was the best scene of last night's episode, Jaime laid his motivations bare to his prisoner, Edmure Tully:
I love Cersei. You can laugh at that if you want; you can sneer, it doesn't matter. She needs me. And to get back to her, I have to take Riverrun. I'll send for your baby boy, and I'll launch him into Riverrun with a catapult. Because you don't matter to me, Lord Edmure. Your son doesn't matter to me. The people in the castle don't matter to me. Only Cersei. And if I have to slaughter every Tully who ever lived to get back to her, that's what I'll do.
Jaime is possibly the most interesting character on Game of Thrones. He started as the despicable, sister-fucking man who pushed a child out of a window. But one of David Benioff and D.B. Weiss' greatest accomplishments is how they've been able to turn the perception of Jaime after his season one crimes. With Brienne of Tarth by his side, he became an actual human being, someone we could empathize with. He lost his good hand, distanced himself from the machinations of his terrible family, shifted his alliances, and showed real selflessness moral fortitude.
It's been interesting to see him slide back into his old ways and self though. As his
nephews and nieces children are cut down one by one, he's re-pledged his devotion to his sister Cersei. "We're the only ones in this world" has become a repeated declaration over the course of seasons five and six, and Jaime's baby-catapulting speech last night reaffirmed that idea. We can tell deep down that Jaime understands the world is more complicated than that, but his love for Cersei—and his fear of leaving her—has been strong enough to make him repress that realization. So since Brienne left King's Landing, Jaime has been forced to choose between reason and loyalty—the former being the harder, more correct choice—and he's opted for loyalty every time. And if a number of Game of Thrones' cautionary tales are to be taken seriously, that's going to be a death sentence for Jaime.
Westeros is not a place for people who refuse to adapt. Ned Stark put nobility above all else, and he lost his head. Stannis Baratheon put himself entirely behind a belief and religious prophecy that he was the "one true king," going so far to burn his own daughter to death, and was he resolutely squashed. Speaking of squashed, Prince Oberyn Martell found out what happens when something like revenge is the only thing driving you. Loyalty, love, belief, trust, revenge—these are dangerous motivators that can blind you (sometimes literally, right Arya?) if you don't check yourself. Ultimately, the only survivors in Westeros are the people who can put their personal goals aside, see the trees for the forest, and change at the last minute.
Littlefinger, Tyrion, Varys, Jon Snow—these characters are more likely to make it to the end because of their ability to reconsider former positions, alliances, and desires. Brienne of Tarth, the great oath keeper, is even more adaptable than she seems, prioritizing her oaths at the Battle of Winterfell in season five and ducking out from Riverrun in a tiny boat last night, even as she vowed to Jaime that, "honor compels me to fight for Sansa's kin."
Unless Jaime comes to understand that there are no rules that can't be broken, and that there has to be some wiggle room in that "us versus them" attitude he and Cersei have adopted, he's going to be dispatched by sharper minds who have a more realistic understanding of how this world works.
He's presumably on his way back to King's Landing to "help" Cersei, and the choices he'll need to make will only grow more difficult. Like he also told Edmure (emphasis is mine): "[Catelyn Stark] loved her children. I suppose all mothers do, but Catelyn and Cersei, there's a fierceness you don't often see. They'd do anything to protect their babies: start a war, burn cities to ash, free their worst enemies." If Bran's visions of wildfire are to be believed, and if Qyburn's talk last night of rumors is referring to what I (and many) think it is—that there's a cache of wildfire in the depths of the city—Jaime may have to do what he did the last time someone threatened to set King's Landing aflame. The Kingslayer may have to become the Kinslayer, he may have to choose between his sister and his one remaining son, Tommen. Or, he can choose to adhere to his hard-line declaration from last night, and burn with the rest of them.