Full spoilers follow.
The origin of the family. Seok-tae, Ki-tae, Jin-seong, Dong-beom and Beom-soo — the gang known as the Day Breakers — grew up together at the Sungji Orphanage, where Im Hyung-taek, the son of the orphanage's director, helped out. Years later the five men targeted Im Hyung-taek's own child for a kidnapping. The operation went wrong, and rather than returning the boy they brought him home and raised him as their own. That boy is Hwayi. His entire life is the byproduct of a botched crime against the family he was born into.
The first kill and the reveal. Seok-tae judges Hwayi ready and brings him along on a job, forcing him across the line into killing. The aftermath delivers the revelation the household had spent sixteen years suppressing: Hwayi learns the truth about the crime that produced him and about the people he came from. The hallucinated monster that has stalked him is retroactively reframed — it is not a supernatural intruder but the shape of what his fathers put in him, surfacing at exactly the moments they trained him to act.
The unravelling. Hwayi turns on the Day Breakers. Because every one of them contributed a skill to his making, his campaign against them is also a demonstration of their curriculum; the film's cruellest irony is that the boy is only able to destroy his fathers because they equipped him to. Meanwhile Seok-tae's response exposes the truth of his paternity: he does not want a son, he wants an heir who cannot exist independently of him, and he would rather destroy Hwayi than lose authorship of him.
The climax. In the final confrontation Seok-tae kills the detective who has been closing in, and Hwayi kills Seok-tae. The patricide is the structural centre of the film's argument — the boy achieves autonomy through the single act that most completely confirms his upbringing.
The ending. The film closes with Hwayi sniping the CEO of the rival gang and then vanishing into the crowd. This is the crucial beat. Having destroyed the men who owned him, Hwayi does not stop; he continues to kill, and then disappears among ordinary people. Read one way, this is the monster having finished swallowing the boy — the title's verb completed, with Hwayi now indistinguishable from any stranger on the street. Read another way, it is a boy with no self left except the one he was given, using it because it is the only thing he has. The film pointedly withholds any final expression that would tell us which.
What clarifies on rewatch. The early domestic comedy becomes unbearable once you know the household is built on a stolen child; the affection between the five men and Hwayi is real, which is precisely why it is monstrous. Seok-tae's shooting lessons read on a second viewing as a father deliberately building the instrument of his own death. And the monster's appearances chart Hwayi's trajectory with more precision than the dialogue does — the director has said the film is seeded with elements meant to look different on each viewing, and this is where they concentrate.