Why did Ki-woo go down to the basement?

Question

During Da-song's birthday party, after a brief conversation¹ with Da-hye, Ki-woo takes the scholar's rock with him and goes down to the basement. What was his motive?

It wasn't entirely clear from the movie.

From what I searched on the Internet, the most popular take on the motive seems to be "To kill Guen-sae and Moon-gwang." However, I'm unable to accept this because:

1. He doesn't necessarily require the rock to do the job. After all, a big rock is not a good option when you're not strong enough to effectively wield it and your plan is to kill two strong adults. Couldn't he have used one of the knives in the kitchen instead?
   Also, if his primary motive was really to kill the couple without getting himself killed, he would have instead ran back when he dropped the rock while climbing down the stairs and come back better prepared with a knife from the kitchen. But, he decided to go on, regardless.
2. Both, his demeanor when he noticed Moon-gwang's body on the floor and the tone of his voice when he says "Hey, are you okay?" twice, suggest that he was genuinely worried about the possibility that she was dead.

The alternative take is "He wanted to pass on the scholar's rock to the couple as a lucky charm." I'm unable to accept this as well because of one reason:

1. Why was he sneaky?
   At the very least, when he dropped the rock, couldn't he have announced (with appropriate level of loudness) "Hey, I come in peace. I'm not here to kill you. I just wish to help you."?

I understand that certain things in the movie can be left for subjective interpretation (such as what the scholar's rock represents). However, I don't believe "The motive behind why Ki-woo went down to the basement with the scholar's rock" is one of those things.

Related questions on SE:

1. What is the significant of the rock?: Both the asker and the answerer of this question believe that Ki-woo's motive was to kill the couple.

---

^₁ The following is the conversation they had before his departure. This describes the thought-process that was running on his mind before he went down to the basement.

Da-hye: You were thinking of something else.
Ki-woo: What?
Da-hye: While kissing me, you were thinking of something else.
Ki-woo: No.
Da-hye: What do you mean, no? You are doing it now.
Ki-woo: Wow... Everyone looks gorgeous, right? Even for a sudden gathering, they're so cool. And look so natural... Da-hye, do I fit in here?
Da-hye: What?
Ki-woo: In this setting, do I fit in?
Da-hye nods (weirdly)
Da-hye: Hey, where are you going?
Ki-woo: I'm going down.
Da-hye: Just stay with me.
Ki-woo: I need to go down there.
Da-hye: Why go to those boring people? Can't you stay here?
Ki-woo: Not to those people. Down lower.

Answer 1

Here's my take on it.

Consider the chronology of the conversations/events about the stone in the end.

Before the conversation with Da-hye, Ki-Woo is having conversation with his father on the gym floor (where the flood-affected people were sleeping):

Ki-woo: I'm sorry.

Ki-taek: For what?

Ki-woo: Everything. All of it. I'll take care of everything.

Ki-taek: What are you talking about? Why are you hugging that stone?

Ki-woo: This? It keeps clinging to me.

Ki-taek: I think you need some sleep.

Ki-woo: I'm serious. It keeps following me.

Then the conversation with Da-hye follows where his face is clearly sad and confused when he asks her whether he fits in such rich environment. That's when he decides to go downstairs and take care of everything as told to father.

At this point there are 3 possibilities of why he went to basement:

1. Kill the basement folks OR

2. Pass on the stone to them OR

3. Perhaps just go there and reach a truce but also carry the stone for safety (as it keeps clinging to him) in case things go wrong.

Given his body language and calm composure the killing option seemed unlikely. My best guess is option 3.

Answer 2

To kill Moon-gwang (former housekeeper) and her husband, Geun-sae

[...] the most popular take on the motive seems to be "To kill Guen-sae and Moon-gwang." However, I'm unable to accept this because [...]

That popular take is supported by the screenplay. According to the script, Ki-woo attempted to kill (finish off by striking) Moon-gwang (former housekeeper) and her husband, Geun-sae (emphasis mine):

INT. MANSION - SECRET ROOM - DAY

[...] he carefully makes his way in, eventually finding --

A tied-up Mun-Kwang fallen next to the toilet.

Ki-Woo swallows nervously. His legs grow weak and his eyes brim with tears as he slowly walks up to Mun-Kwang’s head. He raises the stone to strike Mun-Kwang but --

He can’t do it.
[...]

^{Screenplay source: ‘Parasite’: Read The Script For Bong Joon-Ho’s Provocative Awards-Season Darling — Deadline}

That part of the script is supported by what Bong Joon-ho (director of Parasite) said about the stone (emphasis mine):

“My father, who’s passed now, he used to collect them,” Bong says. “We would go to mountains and streams, and search for stones. It was quite a trend in my parents’ generation, but my son’s generation probably don’t know what scholar’s stones are.”

He adds, “A stone carries this uncanny sense of being able to transform into various things. At once, it can be a weapon, and then the next, a beautiful decoration. You could say this film is about transformation, where these characters transform into fake college students, fake art therapists and fake housekeepers.”

^{Source: Bong Joon Ho discusses the making and meaning of Parasite — Dazed}

Ki-woo, at an earlier point in the film, was willing to use the stone as a weapon to assault the man urinating near their home. It's not out of character for Ki-woo to use the stone as a weapon later.

As a counterpoint to another answer which states that:

At this point there are 3 possibilities of why he went to the basement:

1. Kill the basement folks OR
2. Pass on the stone to them OR
3. Perhaps just go there and reach a truce but also carry the stone for safety (as it keeps clinging to him) in case things go wrong.

Given his body language and calm composure the killing option seemed unlikely. My best guess is option 3.

Item numbers 2 and 3 are not supported by the script and what Bong Joon-ho said about the stone. Ki-woo wouldn't have attempted to use it as a weapon to strike Moon-gwang if he meant to pass the stone to her and her husband, or reach a truce with them. This leaves us with item number 1.