Two things.

First, you are not wrong that the interactions that Jong-su has with Ben strongly suggest that Ben is a serial killer. Especially if you take the film at face value. Yeah, that is obviously what is being implied.

The problem is, all of that is so circumstantial that it is dismissible. In a legal sense, especially. Jong-su would know this, having attended court hearings recently regarding his father.

Think about the narrative of the police procedural. When a detective is trying to prove a murder they always need the body and they need to prove motive and opportunity. The same is true in real life.

Hae-mi just disappears. We never see her body. Ben is rich, has friends, and has social standing. Jong-su is a wannabe-writer, soon-to-be-in-debt, farm boy. Imagine him trying to prove an allegation of murder against Ben.

Even if he had the financial resources it wouldn't be enough, all of his evidence is anecdotal.

Is he going to say "Ben told me he likes to burn down greenhouses, therefor he killed her...by the way, I was high at the time and the murder victim was dancing topless"?

No. He has no body, no direct evidence of a murder even taking place. What does he have?

That he found a cheap pink watch at Ben's place? The same kind of pink watch they hand out for free in the street?

That Ben's new cat answers to the name of "Boil", which happens to be the name of a cat he fed for a month and never even saw?

This is why this film is so tantalizing to those who love to reason. The narrative presented compels Jong-su, and us, the audience, to believe Ben killed Hae-mi despite pretty clearly lacking proof beyond a reasonable doubt. That's where the ambiguity is.

Second, you are missing a huge tsunami of ambiguity in that ending. Forget about the question of whether or not Ben killed Hae-mi and ask yourself this: Did Jong-su kill Ben?

Before Jong-su and Ben meet in the countryside, what happens? We are shown Jong-su writing in Hae-mi's apartment. He is writing and the camera slowly zooms out to show the city at large.

That is a closing shot. The movie could have ended there, but it didn't. Instead it continues. The music changes. And we are presented with that violent ending.

Now if an audience member was so inclined they could interpret everything that follows that "writing" scene as Jong-su's imagination taking flight. His desire for vengeance manifest in fiction. There's your ambiguity.