Theon Greyjoy's redemption arc may be coming to a close on Game of Thrones. By pledging loyalty to Bran Stark, he's pretty much dooming himself to die sooner rather than later. Essentially Theon promised to die for Bran on Game of Thrones in order to right one of his biggest wrongs. But it's not going to end well.
While Theon Greyjoy was greeted at Winterfell with hugs and soup, he didn't come in search of a happy ending. He more or less came to die fighting with the family that raised him. The Greyjoy seems aware that he'll have to sacrifice his life, and the fact that he's choosing to protect Bran is — in its own way — kind of perfect.
As Winterfell prepared for battle, various characters stationed themselves either in the crypts for protection or armed for the fight. Bran knows that the Night King is after him and will try to find him first. "He wants to erase this world, and I am its memory" says Bran of the Night King, finally giving us a hint as to the Big Bad's motivation. Bran practically offers himself up as bait, telling everyone that he'll wait for the Night King in the Godswood by the Weirwood tree, and Theon volunteers to be his bodyguard.
It's significant that Theon put his life at risk to protect Bran in particular. In a misguided and evil attempt to do right by House Greyjoy he abused, exiled, and claimed to have murdered both Bran and Rickon back at the beginning of the series. He killed two innocent boys to maintain that lie. He owes this to Bran. "I took this castle from you," Theon says to Bran. "Let me defend you now." Bran seems cool with it. But Bran's cool with pretty much everything now, but still, he gives Theon and all-knowing nod.
This decision is also significant because the Godswood is where Sansa and Ramsay Bolton were married — a major turning point for "Reek" that lead to him reclaiming his identity and his own morals. All of the poetic justice is lining up for Theon to die very, very soon.
Of course, this could all be a red herring. For one thing, Game of Thrones is nothing if not full of surprises. And anything could happen in the episodes to come; between all of the tearful goodbyes and reunions and people foolishly deciding to hang out in the crypts with a bunch of dead bodies when a dead army is coming, pretty much everyone is setting themselves up for doom. Theon's final act could be a beginning, for all we know.
Theon grew up as Ned Stark's ward, after being taken from his family. His attempts on Game of Thrones to reclaim his Greyjoy side didn't always go so well. Denying his place in the Stark family was Theon's first and biggest mistake. What Ramsay did to him was more than enough punishment, by far, but maybe some of that lingering Stark sense of honor is compelling him to keep trying to right his wrongs to the bitter end. He may die doing it, but formed and chosen families are truly a theme in Game of Thrones Season 8. Nobody's family history is flattering, and when the Night King comes, love is more important than blood.