Smell is an important element in the film. Mr. Park seems to have an aversion to the Kim family's smell, likening it to the smell of poor people. ("People who ride the subway have a special smell.") Ki-Taek and the rest of the Kim family notice this aversion, which mirrors the class difference between them and the "elites," like Mr. Park. At the end of the film, Ki-taek had enough of the "elites." Ki-taek just witnessed his daughter get stabbed, and no one from the "elites" — either the party guests or the Park family was attempting to help his daughter. In desperation, the final straw for Ki-taek was Mr. Park's reaction of disgust to the smell of a "poor person", Geun-sae (the man living in the basement).
According to an interview of Bong Joon-ho (director of Parasite) (emphasis mine):
Question: Smell is a pivotal force in the film. I know it’s the most evocative sense, but are there any other reasons why you wanted to use it to push the narrative forward?
Bong Joon-ho: The jobs that these characters take—tutoring, housekeeping, and driving—feature a rare moment where the rich and poor are together in a very private space and so close to one another that they can smell each other. It was kind of the perfect device in the story.
Source: Parasite Director Bong Joon-ho on the Art of Class Warfare — GQ
Parasite writer-director Bong Joon-ho thinks about the trajectory of his protagonist—the patriarch of the Kim family, Ki-taek—like a volcano. Midway through the class thriller, Ki-taek finagles a job as a driver for the ultra-rich Mr. Park. He hears Mr. Park comment on his unsavory smell, and “he begins to feel more and more pressure until the very end,” Director Bong says. “The last 30% of the film is basically just the magma continually boiling and gurgling until it reaches that explosive moment.”
Question: That scene manages to be both a bit ridiculous and funny, but also very intense. How did you thread that needle?
Bong Joon-ho: I'm grateful for that question because that's what I wanted to achieve with that scene. That man in the basement comes out with the knife in his hands, but even before then he basically cracked his head with the scholar stone, so the violence had already begun. But when the man first comes out into the sunlight, at first he sort of hesitates and almost is shy. Even in my storyboard, I added a note that he should seem like this introverted killer. So it's an absurd moment that's also pretty funny too.
The next thing that happens is he runs with the knife, stabs the daughter, and the violence explodes from that moment on. Everything happens at a very fast speed, almost too fast for the audience to recognize what's going on. It's like a whirlwind. And then, the next moment is the meeting between Mr. Park and the man in the basement. It finally happens and it's a pretty absurd moment. He gets stabbed with a sausage skewer on his side, and even amidst all that pain, he shouts "Respect, Mr. Park," so that's very funny but also sad. And then, because of the smell of this man, Mr. Park holds his nose, and I remember telling the actors to look at the man like he’s this stinky bag of food trash, and that's very cruel. And Mr. Park's reaction has to be that intense for it to act as a trigger for Ki-taek.
Source: Parasite’s Wild Ending, Broken Down — GQ
Da-song: It's the same! They smell the same! [referring to Ki-taek and Chung-sook]
Mrs. Park: What are you talking about? Go up to Jessica.
Da-song: Jessica smells like that, too.
Mr. Park: Where's that smell coming from?
Mrs. Park: What smell?
Mr. Park: Mr. Kim's smell.
Mrs. Park: Mr. Kim?
Mr. Park: Yeah.
Mrs. Park: Not sure what you mean.
Mr. Park: Really? You must have smelled it.
That smell that wafts through the car, how to describe it?
Mrs. Park: An old man's smell?
Mr. Park: No no, it's not that.
What is it?
Like an old radish?
You know when you boil a rag? It smells like that.
But that smell crosses the line. It powers through right into the back seat.
Mrs. Park: How bad can it be?
Mr. Park: I don't know. It's hard to describe. But you sometimes smell it
on the subway.
Mrs. Park: It's been ages since I rode a subway.
Mr. Park: People who ride the subway have a special smell.