Because the twist here is that the President-elect, almost assassinated, saved only by Carrie's and Peter's heroism, and Peter's sacrifice of his life, has turned into a Trump-like paranoid in the White House, arresting not only Dar, but Saul. President Keane, of course, has reason to be paranoid - she was almost assassinated - whereas Trump has none. But, even so, her transformation is profoundly unsettling, to say the least.
Dar, in prison, admits that he did a lot of wrong things - things that set in motion the assassination attempt, which he didn't want to happen. We know that because he was telling Carrie the truth in those frantic moments on the phone - that getting her out of the hotel was a ruse, which would cost her her life. And Carrie listened to him, believed him, and the President-elect survived.
Dat also says he saw something off in Keane, something not right, which is why he was conspiring to what - somehow get her not to get into the Oval Office, without killing her. We don't know exactly what he intended, but apparently he was right about Keane.
Peter getting killed was grievous. And, now, his heroism seems undilutedly justified only for saving Carrie. He has been a great character, even this season, especially this season, and he'll be missed.
And so the curtain comes down on Season 6, broadcast against the stranger reality we inhabit. Carrie will have her work cut out for her in Season 7, with Saul arrested, Dar (who turned out not to be 100% bad) in jail, Peter gone ... and Keane turning into Trump on steroids.
See ya next year.
And see also Homeland on Showtime ... Homeland 1.8: Surprises ... Homeland Concludes First Season: Exceptional