From reading the books, I have always thought, as many others did, that Jaime will end up killing Cersei and fulfilling the Valonquar prophesy. And that they would die together. And I think I know why D&D decided to go with the die together part but not with the Jaime killing her part.
I'm an avid reader, have been so since I learned to read, and there are some patterns and story arcs that you just know before the author reveals them. While reading the books, I guessed many of the developments that came to pass (R+L=J, for example), some developments I didn't guess until the characters did (Joffrey engineered Bran's assassination attempt), and I thought some developments could go in several different directions (i.e. the information provided was not enough to guess).
In the matter of Cersei's death in the books, I'm still convinced it will be Jaime who kills her. But it will happen a lot sooner than in the show, probably in the next book (if Martin ever finishes it). Since D&D loved Lena (and Cersei) and wanted to keep her character around until the end, they could not have Jaime kill her in the penultimate episode. Why? Because Jon will kill Dany in the finale, and we can't have two guys killing the queens they love because they have become monsters in back to back episodes. Jaime killing Cersei would diminish the impact of Jon killing Dany.
If I am correct, then this is one more character development moment sacrificed at the altar of their weird Cersei adoration (and I'm not even a Jaime fan!). Sigh!
I think it's sad that the show will not stick the landing. But, oh, well. life goes on!