He's been getting screwed over completely, denied everything he feels he deserves and robbed of what he thinks is important, his family rules the entire realm and yet he doesn't seem to have an ounce of control over anything.
That's Tyrion you're talking about not Jaime. Jaime had everything handed to him twice, turned it down twice and got his hand handed to him once.
I find it entirely in character as well, considering the circumstances and the fact that he is not and never has been a good dude.
This is not about finding who is a good guy or a bad guy, because there are no good/bad guys in GoT. There may be mad guys like Aerys and Ramsay sure, but the people like Jaime don't just go around pushing kids off buildings for fun - they do it for a reason, and they have a code. When Jaime and Cersei have sex in the sept it's fucking complicated. It is the last time they get intimate and it's the moment their relationship starts going downhill.
The two things in his life that were important to him, that mattered to him - Cersei and his swordarm - have been taken from him. He realizes one is gone for good and the other will never be given to him again willingly, so if he wants anything he's going to have to take it.
On top of that, he legitimately kind of hates her now in a way.
And it sort of happened in the book, but it ended differently.
He's gotten over his swordarm and has gone from being one of the show's most interesting characters (getting the audience to like you after the first thing you did was fuck your sister and throw a kid out a window is not an easy thing) and of all things starts with Jaime abhorring rape - he defends Brienne, and for the first time we're seeing that maybe he isn't a 1 dimensional villain after all. The scene in the bathtub literally made Jaime more honourable than Edd fuckmothering Stark, because he was willing to throw away his entire prosperous future just to save King's Landing from Aerys.
In the books while it doesn't start off consensual, Cersei enjoys it and spurns him on. Not only that, but Cersei is in control here; she still has the ability to tell him to eff off at any moment because hey, Jaime isn't cruel.
Except now he is.
It just seems so out of character that Jaime would suddenly turn his back on her this violently, in the books it is a gradual thing fueled by Cersei's paranoia and Jaime's lost sense of identity coupled with his strong conviction for honour. Now? Simple spite. Jaime's a dick, Cersei's a dick, the end.
It just seems like in their attempt to make the show even darker (because having the Thenns' eat some boy's parents in front of him wasn't dark enough) they've made a blunder that could change the final destination of Cersei and Jaime's character arcs from both a standpoint of interest and grimderpness for the worst.
Meh, they changed something from the book*, therefore it is bad and horrible and all those other things.
*was it, I don't remember. It's been a while.
Oh quit your fanboyism. The show has changed a lot and we talk about how awesome it is, Podderick just now is one such example and the Thenns are another. As much as I hate to say it, I'm actually starting to like the show Stannis more than the book stannis - despite book Stannis being better show Stannis seems more well-thought out and interesting. They made him more ruthless, and so why can't they do the same here?
Oh yeah, because this is Cersei getting raped by Jaime during their last sex scene ever. At least with Stannis they show how apologetic he is to his wife and that in spite of him breaking his vows because tits, he still retains his sense of justice and honesty. There's nothing in the show that gives us the possibility that this would be something Jaime would do, it just appears out of the blue that suddenly Jaime wants to burn his last bridge in the most blunt way possible. There's no dramatic irony from Cersei following in Robert's footsteps to cause Jaime and their's relationship to fail much like history or Jaime steadily shifting away from lover to adviser and from adviser to ignored adviser.
Because we rapists now.
...
I'm just going to remind everyone that in the books Cersei is screaming about how Jaime is home now.
Really changed their arcs for the worst.
And you just know the show is probably going to follow Tyrion's emo phase by the letter now, not changing the one thing that could actually benefit from a character arc change.