For most of the season, Claire didn't bother me. We've seen she has little seeds of doubt for the past few seasons, so it made sense to see some of those start to blossom. It's been obvious that, at some point, Claire would be another person that would lead to Frank's downfall, and her having a crisis of conscience is a good way of doing that. The problem is they just advanced Claire's part in this way too fast to the point where she became inconsistent.
The last few episodes, rather than giving more insight into Claire's motivations, made her earlier actions look illogical. She wants power handed to her, but feels "sick" that she has to ask for it. She is disappointed Frank isn't doing more to help her gain power, but has some deep-seated doubts about the morality of the decisions required to gain that power. Whereas the beginning of the season started to make her look like she was having doubts about what her ambition requires ("We're murderers."), by the end, she's upset because her ambition isn't being stoked enough. Either Claire is walking away because she does not like what she's becoming, or she's walking away because she isn't becoming what she wants fast enough, but it's hard to tell which is the motivation.
Really, I think Claire should've been one of the last people to turn on Frank. She's been his partner-in-crime, the person complicit in taking down a presidency, someone who is willing to do whatever it takes. The longer they had her stand by Frank and continue to feed their ambition, the bigger impact it would've had when she finally did turn on him. There has always seemed to be a part of her that is uncomfortable with what they do, but the catalyst in bringing that to the forefront felt too forced. Her leaving like this really didn't have the sting it should've because Claire crumbled and is so distant from the strong and shrewd Claire we saw in earlier seasons.
You know, people can like a character and watch these shows without respecting or "celebrating" their actions. Don't act all superior just because you are unable to see that complexity.