It wasn't the monsters that unnerved them, but something about the haunted house itself. The place amplified their fears and dulled their senses, leaving them vulnerable.
Even with his rudimentary understanding of the haunted house's layout, YuWon couldn't shake the anxiety each time he encountered forks in the path, traps, or lurking monsters.
Every decision felt like stepping onto a landmine that would blow their faces. But they didn't even have time to think. Monsters would be right after them, and if they hesitated before deciding, they'd get killed.
Once YuWon took a path and saw Anna's face, he was stunned for a moment, until a monster jumped from behind, killing him.