Doug Stamper's (Michael Kelly) mental health deteriorates even further in season six. As Frank's minion of choice, Doug is always loyal but often gets carried away by emotion and acts impetuously (as evident by the murder of Rachel in season three). Season six Doug is no different. The death of Frank clearly is affecting him, but he begrudgingly works alongside Claire, whom he loathes.
Even though Doug agrees to help Claire, every chance he gets in this season he undermines her and actively seeks to make people distrust the president. While he is talking to the press about how he doesn't see eye to eye with President Hale, Claire is also bad-mouthing him in press briefings. It's confirmed that Doug was to be gifted everything after Frank's death, after a last-minute change to his will. He spends much of the season trying to get anyone, especially Claire, to stop maligning Frank and Frank's legacy.
In the final minutes of the season, it's revealed that he is actually the one who killed Frank in an effort to protect the Underwood legacy, since Frank had been determined to kill Claire. After that revelation, Claire is unwilling to acquiesce to Doug's demand that she admit that Frank played an integral role in making her who she is. He gets angry and threatens her with a letter opener in the throat, but when he relaxes, she turns it on him and stabs him in the stomach. He dies on the floor of the Oval Office in Claire's arms.