If one looks at the movie as an allegory of imperialism/colonialism, the interpretation of this scene becomes more complicated.
The linked essay's basic idea is expressed early on:
As Korea’s present colonizer, the United States is implicated throughout Parasite. No single character exemplifies Americanness definitively. Rather, Americanness is an aspirational status. The United States’ presence is thereby marked by its absence, which paradoxically illustrates the totalizing nature of its hegemony. This is most immediately established through the use of English.
The essay gives numerous examples where the use of English points to the status of the characters:
Whereas the Kims live at the mercy of English and the economic system it represents, the Parks’ prestige within that economic order is marked by their proximity to the language.
The use of English points to a larger issue:
South Korea’s capitalist “miracle” was made possible by brutal military dictators who ruled in service of US strategic and financial interests, often with the direct collaboration and knowledge of the US military
Going back to the movie, an important exchange happens just before the party derails:
Dong-ik explains that when Jessica presents the cake to Da-song, the two men will leap from the bushes as “Bad Indians,” giving the birthday boy an opportunity to save Jessica as the “Good Indian.” In a final appeal to some semblance of common humanity, Ki-taek remarks that Dong-ik is also “trying [his] best” to make his family happy. Dong-ik, who has spent the film expressing increasing irritation at Ki-taek “crossing the line,” rebuffs him harshly: “Think of this as part of your work, okay?”
This moment provides the most straightforward portrait of who Parks are. Dressed in appropriated regalia which celebrates and naturalizes an ongoing genocide, Dong-ik orchestrates a sanitized reenactment of settler colonialism through the labor he exploits from the Kim family. The production assumes the perspective of the settler, mimicking the elimination of Indigenous peoples for entertainment. This scene clarifies that Dong-ik’s allegiances lie entirely with the colonizer, in whose name he upholds a capitalist system underpinned by military occupation.
Before discussing the ending, I need to expand a bit on the role of the previous housekeeper's husband who resides in the underground bunker:
The specter of war represented by Geun-sae and the space of the bunker are crucial to interpreting the film’s climax. The ongoing war in the Korean peninsula, sometimes called the Forgotten War, is often narrativized as “over” in a manner reminiscent of how the colonization of the Americas is regarded as complete. The recognition of either process as unfinished undermines the solvency of ruling class power, even as that power is sustained by an endless cycle of colonial violence.
Don't forget that Geun-sae is the one who operates the "automatic" lights:
Meanwhile, in the bunker, Ki-taek finishes restraining an unconscious Moon-gwang, and then finds Geun-sae singing praises to a magazine photo of Dong-ik. Geun-sae shares that he does this every day, and even sends messages of thanks in Morse code through a set of light switches in the bunker. The “automatic” lights on the house’s kitchen steps are actually operated by Geun-sae, who patiently listens for the sound of footsteps overhead as his signal. This revelation further demonstrates how Geun-sae’s silent, hidden suffering maintains the Parks’ comfort.
Now look at his final moments:
Face to face with his Dear Leader for the first time, Geun-sae looks up and greets Dong-ik, who responds with casual disregard: “Are you somebody that I know?” To this, Geun-sae screams “Respect!” in English. Dong-ik is as unmoved by Geun-sae’s dying adoration as he is by Ki-jung’s death, and he turns Geun-sae’s body over without hesitation to get to the keys, pinching his nose shut at Geun-sae’s odor.
And that is what sends Ki-taek over the edge:
The camera lingers on this gesture before showing Ki-taek’s stunned face. Dong-ik has spent the entire film complaining about Ki-taek’s smell, which Ki-jung identified as the smell of mold from their semi-basement home in an earlier scene. The emphasis on olfactory disgust in this moment reemphasizes Ki-taek and Geun-sae’s commonalities. Faced with the war raging beneath his feet for the first time, Dong-ik’s immediate reaction is uncompassionate and self-serving. With the war bonnet still on his head, his callous reaction to the deaths of Ki-jung and Geun-sae is linked to his allegiance to the racist empire.
(Note that much of the previous paragraph refers to ideas expressed at length earlier in the essay. I've tried to summarize them above, but I could only fit in a fraction of the author's extensive arguments.)
This is where Ki-taek breaks, and the specific choreography of what unfolds is key to understanding the action. As Ki-taek lunges for the knife, he tears the war bonnet off of his head and rushes at Dong-ik, whose back is turned to him. Ki-taek grabs Dong-ik by the war bonnet, knocking it to the ground as he turns Dong-ik around, and stabs him in the chest.
Director Bong is known for meticulously storyboarding his scenes, so it’s likely that these details are premeditated. What is it that Ki-taek tears away with his own war bonnet? His complicity? His acceptance of colonial rule? The illusion of the Parks’ innocence? And why does Ki-taek rip away Dong-ik’s war bonnet? In defiance of the racist empire Dong-ik serves? To make plain that Indigenous peoples are not the target of his anticolonial rage? As a rejection of the narrative that the colonial wars subtending our neoliberal illusions of peace are complete? The meanings we could derive are manifold.
The author offers this interpretation:
The irony of Dong-ik dying as a result of his racist assumptions of Indigenous extinction should not be lost on us. In stark opposition to Dong-ik’s original vision, none of the killings are committed by characters in war bonnets. When Ki-taek throws the war bonnets to the floor, he reframes the two sides from “Good/Bad Indians” to “Good/Bad Koreans” — those who serve the empire, and those who are brutalized for its maintenance and expansion. Under conditions of occupation, perhaps being Bad Koreans is the only ethical choice to be made.
The whole essay is very much worth reading and offers a remarkable interpretation of the movie that goes far beyond the superficial.