The pathos of Jaime is that everything he does, including this latest escapade, is driven by one thing, his love for his sister over all else. And for that reason, one thing is certain -- it's not possible for him to have a happy ending to his story. He's accursed no matter what he does. The best he can hope for is a noble ending, perhaps choosing to save Tyrion over Cersei and live or die with a broken heart. And the same for Cersei, the best she can probably hope for is to be slain by her brother. I mean in theory GRRM and/or Weiss/Benioff could just ignore the entire history of tragic drama and allow Jaime and Cersei to skip off into the sunset, but that's not really in keeping with what we have seen in this world. Deaths may seem arbitrary but they usually do have a moral or at least dramatically solid underpinning. Robb done in by arrogance. Catlyn paid the price for her irrational (and turns out completely misplaced) hatred of Jon Snow. Ygritte because Jon broke his vows. Stannis because he killed his daughter. Myrcella cursed by her parentage. Sand snakes, sweet poetic justice.