The Rashomon effect: when the same event is described in significantly different—often contradictory—ways.
It's pretty clear how things ended in the webtoon. But not really in the live-action adaptation. After all, it doesn't make sense that Seo Moonjo said almost the exact same words twice. They couldn't both be real, nor could they both be hallucinations. One had to be real, while the other had to be a mere hallucination—which was practically Jongwoo's attempt to lie to himself. Just like what they said in the Rashomon, "It's human to lie. Most of the time we can't even be honest with ourselves." So Jongwoo hunted down everyone else there, killed them one by one. Then went back to the 4th floor, where Moonjo and unconscious Jieun waited. He freaked out when he realized that he, too, enjoyed killing. Moonjo touched his neck, praised him, and then left him there. To his broken mind, to him hallucinating—and that's when Jieun regained her consciousness, witnessing her boyfriend got lost into his own delusion. Nevertheless, I have to emphasize here that Jongwoo never tried to trick his mind into believing that Moonjo killed all those residents. He was fully aware of what he's doing, and even thrilled when he did it. The only thing he was hallucinating about was him killing Moonjo. Because in his mind, killing him was the only way to end this madness. The only way to 'save himself'.
Now that I think about it again, Moonjo is alive y'all. At first I thought it was fact (pieces of info that support the theory that moonjo was dead) vs assumption (pieces of info that support the theory that moonjo was alive). But they're all actually just assumptions. So if we strip the assumptions off, we get the facts, which are:
1. Wet-haired!Moonjo was Jongwoo's hallucination.
2. The detectives told the whole world (including the thug and Officer Soh) that Moonjo was dead. That's a fact. But their words should NOT be taken as facts.
We can create fucking tons of assumptions only by these 2 facts. But the puzzle fits so fucking perfectly that Moonjo is alive and therefore I am: going mad (/¯◡ ‿ ◡)/¯ ~ ┻━┻
