It's your fault.
You attended Hyosan High School. You were close friends with Lee Na-yeon. In some ways, you were similar to her, selfish and apt to bully others. While you weren't as bad as Na-yeon, you weren't much better either. You were smarter and more cunning than Nayeon and often took advantage of her.
Despite your difficult personality, you maintained good relationships with some of your classmates. You mostly hung out with Na-yeon, but sometimes with Choi Nam-ra. Even though you considered Nam-ra boring and the class outcast, you still hung out with her. In Na-yeon's presence, you would speak unkindly of Nam-ra, knowing that Nayeon and Nam-ra were enemies.
A few days had passed since the apocalypse began. You, like everyone else, were locked in school. Several students had already become infected. You were terrified, but you tried to keep it under wraps and act calm, even though you were genuinely afraid inside, so you always tried to be with someone. It was either one of your classmates or Na-yeon, mostly Na-yeon.
Things weren't going well, but at some point, they got worse. Specifically, one of your classmates, Han Gyeong-su, got infected. It seemed unremarkable, at least for now, but the thing was, Na-yeon was responsible for his infection. She treated his wound with a rag soaked in zombie blood, and she did it on purpose. And most importantly, you knew Na-yeon was going to do it, but you didn't stop her.
Na-yeon hated Gyeong-su, and that's why she did it. Even though Gyeong-su was a nice guy, Na-yeon didn't get along with any of her classmates except you. She hated Gyeong-su more than anyone in the world, and that's why she did what she did. Nam-ra noticed Na-yeon's actions, and although Na-yeon initially denied it, she then lost it and ran out of the classroom. No one has seen Na-yeon since.
Then you didn't go after Na-yeon. You didn't understand why, but you decided to stay with the others instead of running after Na-yeon, as usual. Everything could have been fine, but Nam-ra somehow guessed that you knew what Na-yeon was planning to do to Gyeong-su. Perhaps Nam-ra guessed from your reaction to what was happening, which showed you knew about Na-yeon's plan.
Nam-ra didn't tell anyone that you knew about Na-yeon's plan and didn't do anything, but Nam-ra started staring at you. It seemed like something had changed in her after the infection and Gyeong-su's death. You felt uneasy under Nam-ra's gaze. It was even worse than if she'd accused you directly, because you knew you were to blame, but you tried to convince yourself of your innocence, because it wasn't you. Na-yeon infected Gyeong-su, so it wasn't your fault.
At some point, you lost your composure. You couldn't stand Nam-ra's intense gaze any longer. You stood up from the desk you'd been sitting on and walked toward Nam-ra, muttering something under your breath. You tried to appear strong, hiding your inner turmoil.
"Why are you looking at me like that? Am I the only one to blame for what happened to Han now?"
Nam-ra stared at you silently for a moment. This infuriated you even more, but you still tried to maintain your composure. And yet, after a few moments, Nam-ra spoke.
"It's your fault."
You expected to hear anything but those simple words. This only angered you even more. You tried to restrain yourself, still trying to defend yourself.
"What should I have done? Answer me."
Nam-ra didn't answer, which only further angered you. You suddenly lunged forward, grabbed her by the shoulders, and, almost shouting, demanded that she answer you. But then you turned around and saw that all your classmates were talking about something, probably still discussing what happened with Gyeong-su. They hadn't yet noticed your conversation with Nam-ra. You decided not to continue trying to talk to her and simply stepped aside.