How did they destroy Jaime's character? In fact it is his character in the books who is awful. The only reason he leaves Cersei is because Tyrion says she was fucking around. And what does he do, does he talk to Cersei about this or address the situation at all, no he just wallows in "Lancel, Kettleblack and Moonboy" for chapter after chapter.
How would you characterise Jaime?
What? Book Jaime absolutely takes a dump on show Jaime. Book Jaime the only thing keeping him going is that he wants to get back to Cersei who he feels needs her. And instead she gets disgusted at the loss of his hand. He doesn't reach in time to save Joffrey he arrives just after he is dead. And they have sex in a moment of passion near his body. Show Jaime straight up rapes a grieving Cersei against her consent.
In the book, Jaime is coming to terms with everything he believed and did. He doesn't want Ned's Ice broken down into a new sword for him, he wants to rebuild the Kingsguard and make it what it was. He wants to spend time with Myrcella and Tommen, he feels regret at not helping Catelyn find Arya or Sansa, and goes so far as to write in the white book that his best deed was being defeated by the young wolf so people wouldn't forget Robb who was a good man. He starts seeing Cersei for who she has always been. Once he finds out that while he was only with Cersei, she has been with Lancel, Kettleblack and Moon Boy for all he knows. That she is crazy enough to use wildfire that could have hurt innocents, the very thing he slayed the Mad King for.
In the show, Jaime has a few moments of sincerity like when Joffrey is insulting him about being mid thirties and no hand and won't have the same accolades like his heroes Ser Barristan or Ser Arthur Dayne did. Or his reunion with Myrcella. I also blame the showrunners and writers for this. Cersei is far too sympathetic on the show. They want to utilize Lena Headey so much she has become a victim and people feel for her as opposed to the books which make it clear she is just as power hungry, crazy and has no clue etc. Olenna pretty much says what we should all think about Cersei but I still pity her. Especially the way they framed Tommen betraying her over the trial by combat.