Throughout Parasite, a painfully unsustainable wealth gap means the difference between privilege and dehumanization. Though the Kim family rejects those who they feel are below them, the third act of the film demonstrates that they are below as well, though they didn't understand that. Although they serve the Park family, they are in the Parks' home because they are working to be there. They do not feel that they are leeching off of the Parks and they condemn those who are.

Though they are aware that they are below the Parks, as the wealthy family points out things like their unpleasant and unclean smell, the Kims still establish a hierarchy. When the Parks' ex-maid (Lee Jung Eun) informs them that she had been hiding her husband (Park Myeong Hoon) in the Parks' basement and begs the Kims to allow her husband to stay in the basement, they shun her.

They do not see their situations as equal: one is parasitic, leeching without permission, and the other is working to stay in the Parks' home. That's why the end is so life-altering for the Kim family patriarch, Ki Taek (Song Kang Ho).

After being told that they were to not take a weekend off and instead be working for the Parks' son, Da Song's (Jung Hyeon Jun) birthday party, the party turns to chaos. The man hiding in the basement emerges, wanting vengeance on the family that wouldn't accept that they were "neighbors in need". He issues a blow to the head of Ki Woo (Choi Woo Shik), the Kims' son, and stabs Ki Jung (Park So Dam), their daughter. 

The man is taken down, but Ki Taek is left being commanded by the Parks to take their son to the hospital after he fainted from the drama, while his own daughter is bleeding to death. While that might be enough to make Ki Taek want to kill his employer, Bong took it a step further. Dong Ik (Lee Sun Kyun) couldn't resist commenting on the housekeeper's husband's smell, even after the man was dead in front of him.

It was in that moment that Ki Taek realized, in Dong Ik's eyes and in the rest of the Parks' family's eyes, Ki Taek and his family are no better than the housekeeper and her husband. They live at the whim of the Parks, constrained by servitude, constantly trying to climb up out of the hole society has kept them in, only to be stomped back down again and again. The Kims would never be the Parks and they would never be able to stop living below them, controlled by them. It's in that moment that Mr. Kim stabs Mr. Park.