Kim killed Park at the end because of the reasons well-stated above, I like to add that it perhaps also because Mr.Kim finally saw the true nature of the Park family. The wife didn't even bother to call an ambulance, nor did anyone else at the party, and the husband was still disgusted by the basement guy's smell as he went for the keys. This showed the prejudice and the cold-hearted characters of the so-called upper-class "Elitists".
@Steve K. You didn't seem to understand the movie at all, the sequence of action including the loose ending is intended to be there for people to think, it's an open-ended question, versus praise for killing. The movie made no reference is God, yes the Christian God, and people could interpret it that way if they wanted to. It sounds like you got so offended when you saw a "rich, selfish, and indifferent" person die. Why did Park die? Perhaps because God actually also judged him, for his lack of compassion and basic respect to others. Park thought money is everything, and if I pay you, you better be my bitch... at some point, the person on the other end of the job just won't take it anymore. Because Park never, nor did his wife, respected their servants (The odor is just a symbol, cannot be taken literally. It stroke home for Kim when Park showed Kim that he crossed the line when Kim mentioned to Park about Park's wife when Kim only said it because he thought Park could be a friend. Park never considered Kim, and everyone working for him as equal).
Is living in the basement, becoming another "parasite", losing a daughter and a half a son (brain damage) a good ending? Kim would probably eventually rot to death in the same old basement, without a proper burial. Would anyone consider that a nicer ending versus getting killed?
Living isn't everything, it is more about how you live, whether or not like a parasite...