********MAJOR SPOILERS********
The ending of Knock at the Cabin is built around one awful question: are Leonard and the others delusional, or are they telling the truth about the end of the world? By the final stretch, the movie has stopped treating that as an abstract debate. The family has seen too much. The judgments keep arriving. And the decision can no longer be postponed.
After Redmond, Adriane, and Sabrina are sacrificed, the disasters on television keep matching what Leonard promised. Tsunamis. Disease. Planes falling out of the sky. Andrew keeps fighting the conclusion because he has every reason to. If he accepts Leonard’s story, then he also has to accept that the world is demanding the murder of someone he loves. Eric, though, starts moving in the other direction. The signs are piling up. The scale is getting harder to deny.
Then Leonard reaches the end of his own role in the ritual. He tells the family there is almost no time left. After his death, the final judgment will come unless they choose. He slashes his own throat, and the world outside seems to answer immediately. Lightning. Fire. More devastation. By then the movie has made it almost impossible to hide inside uncertainty the way Andrew has been trying to.
Eric is the one who finally sees the ending clearly. He believes the intruders were right. He believes the family was chosen. More than that, he believes Andrew and Wen still have a future if he gives himself up now. The movie lets Eric imagine that future for a moment, older Andrew, older Wen, life continuing. Then he asks Andrew to do it.
That is the emotional core of the ending. Andrew does not make the choice cleanly or heroically. He makes it in anguish. He shoots Eric because Eric has convinced him that refusing would condemn everyone else. In other words, the family does choose, but only after the film has wrung every drop of certainty and pain out of the decision.
After that, Andrew and Wen go out into the world and check whether Eric’s sacrifice actually worked. They find the intruders’ materials in the truck, which helps confirm that this was not a random home invasion. Then they reach the diner and watch the news report that the global catastrophes have stopped.
So the plain version is this: Leonard dies as the last messenger, Eric accepts that the apocalypse is real, Andrew reluctantly kills Eric as the sacrifice, the disasters stop, and Andrew and Wen drive away into a world that has been saved at the cost of their family as they knew it. The movie closes on survival, but it is survival with a hole blown straight through it.