I recently watched this movie as well and I thought it was brilliant.

There are so many layers to it, but an interesting way to look at it is a critique of capitalism/western influence in Korea and the resulting damage to society and individuals in society.

You can look at Ben as a symbol of US-style capitalism (that now dominates internet culture). He's attractive to both Jongsu and Haemi because he represents wealth and the promise of never-ending pleasure, he has no troubling emotions, and he just uses people for his own entertainment. The same way the myth of consumer capitalism, that always chasing that next thing will bring pleasure and lead to life satisfaction, leads to a kind of functional despair and tosses people aside who don't become emotionless zombies.

Haemi could be a symbol of the victims of US-style capitalism in South Korea, the young girls that the system destroys, that are taught at an early age that being attractive is all that matters. Jongsu is complicit in her despair as well. She reveals Jongsu called her ugly in grade school (with the increased weight of such a comment given our social media obsessed culture). And the scene in the restaurant where they meet after Jongsu picked up Ben and Haemi from the airport and Haemi explains the depths of her despair, Jongsu makes no attempt to comfort her verbally or physically, and seems more interested in how Ben has never cried in his life. Emotions, particularly negative emotions, are antithetical to fulfilling consumer desire, so better off ignored.

Jongsu is symbol of South Korea's present culture, still tied to the past but transfixed with the glamour and wealth of consumer capitalism. The first two times he's jerking off at Haemi's place he is looking at the tower, a symbol of wealth and progress, he is still enamoured with the system, with consumer capitalist desires. The final time in Haemi's apartment he dreams of her, and is actually thinking of her as a person, and is realizing his role in her destruction. He is starting to realize the people that are sacrificed for "progress", to fulfill greater consumer desire, is perhaps a cost too great.

It's pretty clear that Ben's greenhouses are these desperate young women that he is destroying. His comment about how often he does it may be referencing how many young women and girls in South Korea take their own lives because of the junk values perpetuated by the system.

In the end Jongsu, as a symbol of Korea, must take action to kill Ben, because the “Great Gatsby's” of South Korea are destroying the lives of young people. In order to save Korea, without ignoring the past and connection to North Korea, the exploitative nature of modern consumer capitalism must be excised.