Hyun-seo's capture by the creature is Adult Fear realised.
The gangsters Hee-bong buys the truck and the map from are pretty intimidating.
Gang-du getting tissue samples taken from his neck is rather uncomfortable to watch, especially for those who hate hospitals. The most horrific image, though, is Gang-du being strapped to an operating table by doctors, helpless and tearfully begging for the absent Hyun-seo's forgiveness.
The death of Gang-du's father. Particularly when the creature throws him with its tail and smashes his skull on the riverbank.
The book by Stephenie Meyer:
The very concept. What isn't scary about the idea of your entire family, including babies, being taken over by aliens and creating a "paradise" that's almost completely devoid of meaningful conflict or emotion?
Even scarier - just think about being imprisoned in your own body as a Puppeteer Parasite takes over your life, including your love life. Sure, hosts tend to stay in the same job their hosts to avoid attracting too much attention, and they also tend to fall in love with people their hosts would also find appealing, but that's not always the case (Exhibit A: Wanderer). On the other hand, host's personalities tend to fade over time, depending on how strong their minds are, but that isn't always the case either (Exibit B: Melanie). Now, imagine being forced to watch as someone else completely deconstructs your life to fit their requirements and their talents, not yours. Imagine being forced to have sex with someone you did not choose, but "the other you" did. And imagine that, unlike Melanie and Wanda, "the other you" is not your friend. Pardon me, I'll be sucking my thumb in my bomb shelter...
Kyle's attempted murder on Wanda certainly counts. No matter how anyone tries to spin it, Kyle is rather..unhinged, and his attack on Wanda was brutal, and his methods unnecessarily forceful and crude. He tried to drown the girl, and the entire scene was played out as a struggle that Wanda was barely winning. Had the floor not caved in, or rather, had Ian not arrived in time, Wanda could very well have been killed by Kyle over his own inability to let things go.
When a few of the humans start warming up to Wanderer, she has difficulty accepting this, and reminds one of these humans that in reality, she is a small parasitic worm. She tells the human (paraphrasing) "if you could hold me, the real me, in your hand, you'd throw me to the ground and ground me under your heel" — a pretty disturbing mental image in itelf, given that, alien parasite or no, we're talking about a self-aware, sapient being. Even worse, the human has to pause and then replies, "not if I knew it was you" — to which Wanderer responds by pointing out that the human would have no way of telling her apart from any other "Soul". The human has no good response to this. That's... pretty dang dark, and disturbing — the humans certainly have excellent reasons to hate the Souls, but it's still disturbing to see "what measure is a non human" in effect to such a huge extent, especially since the narrative makes it very easy to see the exchange from Wanderer's perspective.
On a happier note, later in the book, this exchange sets up an *awesome* Crowning Moment of Heartwarming when Wanderer's statement is put to the test.
When Ian ask Wanda what would happen to him if he were caught, she answers that he wouldn't be kept as an host because full-grown humans are too stubborn to be mind-wiped, so the new policy is to discard them. And the Souls have no problem with that, because why would you keep a broken car you're unable to use? That's the turning point for Wanda, realising that her human friend and protector doesn't deserve to die only because his will to live is too strong to let him disappear.
However, as Ian is a rebel, she confesses he would probably be used as host... by a Seeker wanting information about other wild humans. For the record, it happened to Melanie's father, who remembers her dad's body leading the Souls to his daughter and son's refuge and smiling while the aliens try to capture them.