An obvious theme is The Haves vs. The Have Nots highlighted by the juxtaposition of the economic standing of the respective families, but I think there’s something more insidious going on thematically and that is this: The love of money is the root of all evil. This is a verse from the New Testament: 1 Timothy 2:10.
The Park family is wealthy and because of their wealth, they have a skewed perspective of reality. They live in cloistered comfort and believe they are protected from the vicissitudes of The World Out There.
The Kim family lives in relative poverty and in their desperation to escape it, lie, cheat, and steal their way into jobs with the Park family. Indeed, they use the Park’s family naivete which derives from their lack of interface with The World Out There to enable their (Kim family) schemes.
What neither family realizes is there is a ‘rot’ at the economic foundation of the Park family existence and, by extension, the Kim’s with their new jobs. This is represented by Geun-sae, the former housekeeper Moon-gwang’s husband who has been living in secret in the Park family house basement for years and years.
The thing is the Kim’s could have successfully pulled off their scam if they hadn’t gotten greedy, if they hadn’t begun to feel as if they deserved to be wealthy. Orchestrating Moon-gwang’s dismissal is the straw that breaks the proverbial camel’s back, setting into motion the movie’s violent ending.
All of it comes back to money, specifically the love of money which leads pretty much everyone astray.