Throughout the series, Jong-woo suspects his neighbors of orchestrating horrific acts while Seo Moon-jo systematically murders anyone threatening the gosiwon’s ecosystem, including inquisitive detectives and uncooperative residents. In the finale (Episode 10), Moon-jo captures Jong-woo's girlfriend Ji-eun to force Jong-woo back into Eden Studio. In a savage climactic purge, the residents turn on each other: Ms. Eom murders the twins, while Moon-jo eliminates Ms. Eom and the other accomplices. When police officer Jung-hwa and reinforcements arrive, they find all gosiwon residents slaughtered, Moon-jo dead with his throat slashed, and Jong-woo hospitalized as the supposed lone survivor.
The Key Twist: Jung-hwa listens to the audio recording and Ji-eun’s traumatized testimony, discovering the truth: Moon-jo had incapacitated the victims, but it was Yoon Jong-woo who systematically butchered every resident in a berserk frenzy, chanting that he would kill them all. In the final hospital scene, as Jong-woo mindlessly types on his laptop, he wears Moon-jo's tooth bracelet, clicking his teeth in Moon-jo’s signature manner, and his reflection shifts into Seo Moon-jo—revealing that the monster’s soul and mission have taken over Jong-woo entirely.