
Architectofnonsence:
I can't put together how both murderers happened at the same warehouse and on the same night and they were entirely diffirent from each other in every detail but that psycho female lead witnessed them both or may be I missed something
So... there was supposed to be a festival that night in the village (in the drama, they're about to get plastered because they just finished highschool.) Everybody was in the village center, then (in-novel) GJW saw NaDeun leaving with someone else, told the others he was going home, got drunk and passed out.
The barn was where they could all go and drink, because they couldn't go to someone else's houses (the other boys and the girls). (IN) SuOh was already hiding close to the barn (not inside) but could see everything, (IN) NaGyeom was also hiding there, mad that GJW was heartbroken over NaDeun. So they too got drunk, got mad at (IN) GJW for ditching them, BoYeong insulted them and rubbed their noses into how little they were compared to GJW, so they raped her while (IN) GeonOh had left to get more alcohol, then when he returned, she was trying to flee, got scared of him and fell, hit her head and they thought she was dead. Then NaGyeom told them to hide the body, THEY (not the parents) carried BY to an old oil tank, closed the trap door on her (she was awake and begging to be let out by then) and ran home. They told their parents and the parents went there and buried the trap door under dirt in order to stop the boys from returning and getting her out, in case they got remorseful.
At this time, before NaGyeom could leave, NaDeun and her lover (he's a teacher at the time, a politician later, the drama did it differently) got to the barn, since she knew it was a safe place to hide and thought the rest of the gang would be in GJW's home, drinking. She was killed pretty much as the drama has shown, then the killer fled, NaGyeom took the weapon and went to leave it at his door, and poor SuOh was left there, nobody wiser to his presence. He could only do what his shocked system could think of - he took NaDeun's body to hide until she'd come alive, or whatever he was thinking.
And then (IN) GJW became the designated killer of two girls because it was just so convenient to cover everyone else's sins.
So you see... a game of coincidences, for a bit.