Thoughts
Throughout the film we're shown how the Parks' family has an aversion to the way everyone smells. The Parks associate the scent of Mr. Kim to poverty. For Mr. Kim, he sees this open disgust of their smell as an affront to their underprivileged circumstances. When Mr. Park attempts to take the keys from Geun-Se, he covers his nose because the smell becomes overpowering again. This is the tipping point for Mr. Kim, because he sees that despite all of the madness, Mr. Park is caught up in the minutiae of how people smell.
Mr. Kim sees this as a sign of extreme opulence--an unnecessary luxury, if you will. Mr. Kim believes that his family is giving more than they're receiving in the form of payment. He believes that the reason the Parks are so kind and welcoming is a function of their wealth.
In the end, human jealousy takes over, and we're presented with the madness in the end. Although I'm not quite certain about the impact Geun-Se and Yoon are supposed to have on the audieince, as that reveal felt a bit underwhelming to me.
I found the Kims to be pretty unlikable. They are, as the film depicts, "parasites", in the sense that they don't appear to put any effort into bettering their situation. They want to take from others and give very little in return. This is seen from the way they fold the pizza boxes in the beginning. Their jealousy is unfounded, as Mr. Park worked for his lot in life. He is the product of a strong work ethic and ambition. Was luck involved in some of his success? Sure. But I like to believe a large part of his circumstance could be attributed to personal agency and the pursuit of self-improvement.
The saddest part isn't that the Kims were living in poverty, but that they had the knowledge and skillset to become meaningful and successful members of society. They wasted a lot of their energy wallowing in self-pity. They could have sublimated some of their anger towards something productive. It's one thing to be content with mediocrity, it's another to hate others who aren't and view them as the root of your problems.
Wasted potential in the end. They programmed themselves for self-destruction, even when circumstances opened up for them.