Hallucination.
You were an ordinary student at Hyosan High School. At first glance, you were completely harmless, though you had your obvious flaws. Perhaps you were influenced by your friend Lee Na-yeon, a spoiled brat from a wealthy family who always looked down on everyone. You were Na-yeon's only friend, as she didn't get along with anyone.
You were slightly better than Na-yeon in personality; at least you weren't an outcast among the students. You had a few other friends besides Na-yeon, but you mostly spent time with Na-yeon and were very close to her. You were the only one Na-yeon treated well. You were almost always seen together. Perhaps you felt some affection for Na-yeon, but always hid it.
The apocalypse had recently begun. You, like the other students, were locked in the school. Despite the current situation, you showed no fear, pretending not to be afraid, even though you were terrified inside, but feigning bravery on the outside. As usual, you stayed close to Na-yeon, who, though she was afraid of what was happening and didn't particularly hide it, still remained brazen and defiant.
One day, Na-yeon committed an act that even you considered wrong, she infected Gyeong-su with a virus using a rag soaked in zombie blood. Gyeong-su was your classmate and a good guy, but Na-yeon hated him and so eliminated him.
When everyone learned that Na-yeon was responsible for Gyeong-su's death, she became even more of an outcast. At first, you didn't even approach Na-yeon because you never expected her to, and she herself probably realized what she had done too late. When Na-yeon lost her composure and ran out of the classroom, you ran after her and managed to catch up with her in the hallway. You told Na-yeon you were on her side, even though you weren't sure anymore.
And then, at some point, as you and Na-yeon were running from the zombies down the hallway, you ran into one of the open classrooms. But when Na-yeon tried to run into the classroom with you, you pushed her aside and closed the door. Na-yeon was clearly in shock, she knocked and tried to open the door, but you held it, preventing her from opening it. A few moments later, you heard her screams, but you still held the door.
After a while, Na-yeon's screams died down. You heard the sounds of the zombies growing fainter and fainter. After confirming that the zombies were gone, you opened the door and immediately saw Na-yeon's body. She was now dead. Her eyes were open, you looked into them and saw nothing alive. You didn't even know what to feel. Your actions weren't very thoughtful. You acted on impulse, but now it was too late to question the wisdom of your actions, because everything was already done.
Now you were still sitting in the classroom where you'd locked yourself in earlier. You had some supplies, enough to last you a few days. You also took Na-yeon's bag, in which you'd found something useful. Despite your desire to forget, you couldn't forget Na-yeon. Over all the time you spent together, you'd truly grown attached to Na-yeon, and now she was gone, and you were left alone.
Lost in your thoughts, you heard sounds behind you. You grabbed the pen and spun around, pointing it at someone. You expected to see a zombie or one of your surviving classmates, but instead, you saw Na-yeon. You couldn't believe you were seeing her. You saw Na-yeon's body. She was definitely dead. You clutched the pen tightly in your hand, but then lowered it.
You noticed that Na-yeon had become slightly translucent and looked slightly different. There were a few tiny bloodstains on her clothes. Na-yeon was smirking, that same smirk you remembered so well. Na-yeon's gaze was different; she'd never looked at you like that before. You stepped back, unable to believe this was real. It was a hallucination, but so realistic. Suddenly, Na-yeon spoke in a cold voice.
"What, are you regretting it now? I'm sorry, but it's too late. You know, I didn't expect this from you. You're even worse than I thought."