Us: What Does The Ending Really Mean?

6. What Exactly Are The Untethered?

Us reveals a surprising amount about about its antagonists from the get go, literally sitting the characters down with their doubles to explain just who the hell they are and where they came from. During that scene, it's revealed that they were "birthed" during some kind of experiment as a means to control the population through brainwashing. The problem was, while the scientists could copy the body, they "couldn't copy the soul", which meant they couldn't be controlled and thus the experiment was abandoned.

The implication is everyone on the surface has a copy of them underground, who live a life of squalor, forced to copy the actions of their doubles above ground. Adelaide's double, Red, is different (which we find out in the final scene is due to her being from the surface). Consequently, she's viewed by the others as some kind of Messiah figure, and leads them to execute the "Untethering", where they'll rise to the surface to kill their doubles and escape their conditions.

Consequently, they're not monsters, or inhuman, or entirely the villains at all, they're simply "Americans", just like the rest of the characters. Of course, that's the literal explanation, and it's their symbolic function in the narrative that's a bit trickier to make out after the climax...