The following contains major spoilers for Us.
Us is a movie that has a lot of audiences talking. It's a horror movie with a lot of symbolism and metaphor that has sent many back to see the film more than once and still left them with questions. However, nothing has more people talking than the film's twist ending. At the end of the story we learned that not everything was exactly what it seemed. According to director Jordan Peele, that twist was necessary because it served the entire theme of the story.
In the final moments of Us we discover that the character that we've known as Adelaide isn't exactly who we thought she was. She had changed places with her "above ground" counterpart and taken over her life as a child. This confusion over who was the "hero" and who was the "villain" of the story was exactly what the story was always about, as the director tells the Empire Film Podcast. According to Jordan Peele...
This movie’s about maybe the monster is you. It’s about us, looking at ourselves as individuals and as a group. The protagonist in the movie is the surrogate for the audience, so it felt like at the end of the day, I wasn’t doing my core theme any justice if I wasn’t revealing that we have been the bad guy in this movie. We’ve been following the villain. I say villain lightly because I think there are many experiences of the film, and I think a lot of people go through a question of what is good and evil? Does that even exist? Both characters are lovable and terrifying, based on the lives they’ve led they’ve just sort of inverted the paths.
Jordan Peele had spoken at length, even before Us was released, about how the idea of the movie was about looking at ourselves and realizing that the true enemy might not be some sort of nebulous "other," but actually ourselves. This is obvious in the fact that the "monsters" of this horror movie are dark and twisted versions of the main characters.
However, this idea is taken to another level when the film's twist is revealed. The audience is forced to question who the real monster is. We now have some degree of sympathy for the character we knew as Red, who had her life stolen from her at an early age. At the same time, the character we know as Adelaide is largely still the same person that we've known through the movie. She's still the same wife and mother trying to protect her family, even if it all came about following a questionable act as a child.
Even if you figured out the twist early in the film, as I was unfortunately, able to do, it doesn't really lessen the impact of the moment. In fact, if anything, seeing where the movie is going early on only causes you to consider the movie's questions for a longer period of time, as you watch the events unfold. That's probably the mark of a good twist.