WARNING: Major spoilers for House of Cards season 6.
Here's who really killed Frank Underwood in House of Cards. Kevin Spacey's character is dead. That much was made clear before House of Cards season 6 had even hit Netflix, but the circumstances of how the show would write out the former-President were less clear.
Netflix and the series’ producers were left with no choice but to retool the sixth and final season of House of Cards after details emerged of his sexual misconduct. Spacey was fired and, after a delay in production, the show went ahead with Robin Wright as the sole lead and a planned eight episodes instead of the usual 13.
Those eight episodes have now landed on Netflix, but the big question is what happens to Frank. That’s not even just in terms of a cultural standpoint, but quite literally the in-show universe: from the very first chapter of the new season, Frank’s death - or more specifically, the person responsible - becomes a mystery that spans the entire length of the show. They can write out Spacey, but Frank’s presence lingers like a ghost over the entire season.
We hear of the official reasons for Frank’s death quickly: publicly, he died in bed next to Claire, and an autopsy report states an overdose from complications with treatment for the live transplant her had. Claire plays the grieving widow, with shots of her at his funeral, looking into his casket (his face is, of course, never seen) but privately she suggests foul play, and that her husband was quite possibly poisoned. As she says early in the season: “A man like Francis doesn’t just die. That would be - what’s the word? - convenient.”
Various suspects are brought in along the way, from suggestions Claire herself did it to her new rivals Bill and Annette Shepherd (Greg Kinnear and Diane Lane) but it’s only in the final 10 minutes we get the truth.
Doug Stamper Killed Frank Underwood
As Claire Underwood, who by this stage is pregnant (the child may be Frank’s, it may be Tom Yates' - Claire herself doesn’t seem sure), and Doug Stamper stand in the Oval Office, she forces a confession out of him. Frank was going to kill Claire for going back on her promise to pardon him, and Doug intervened and ended up killing his mentor.
A visibly shaken Doug admits he didn’t have a plan, but he had to protect the legacy from the man. To the very end, Doug did everything he could, no matter the cost, to protect and serve Francis Underwood. He couldn’t allow him to kill Claire because it would further tarnish his reputation, he "couldn’t let him destroy everything we’d built", and in the end did use his meds to kill him.
House of Cards then concludes with Claire telling Doug to admit he’s happy Francis is gone, after which Doug snaps and puts a letter opener to her throat - the letter opener that Frank bequeathed to him in his will - only for her to get him to back down, and then stab him instead, before suffocating him for good measure. Doug, for his part, doesn’t even try to resist, but seems accepting of his fate, with Claire saying "There, no more pain."
Doug dies, Francis’ reputation is in tatters, and Claire remains in the White House, still pregnant, as the season, and the series of House of Cards itself, fades to black.