Mourning Grave" is aiming for social commentary. Specifically, it's referencing bullying, and how the lack of public acknowledgment of the existence of the practice is what's causing ghosts to come up from the grave and start murdering their tormentors, for the simple reason that no one else is going to do anything about it. The mystery elements in "Mourning Grave" are particularly odd, because everyone treats In-soo's apparent curiousity with a great deal of rudeness. The population only starts addressing the murders when the body count gets too high to be reasonably ignored.