What was the main guys problem?

*SPOILERS*








Yes, I think something like that happened. Either the unscarred sister, be she ghost or human*, infected him with some sort of crazy/paranoid impulse or the house itself did. There's no other explanation other than that he was mentally unstable to begin with, and the events of the night pushed him over the edge.

*One of this movie's flaws, aside from extremely unlikable characters, is that the nature of the revenge killer(s) is never clearly explained. Is the unscarred sister a ghost? Is she real, taking vengeance for her dead/ghost sister? Maybe they're both alive (until they're killed). Note that the story Noemi translated gave almost no details and never said the burned girl actually died. Finding out that there are two of them takes away some of the "ghost" nature of some of the scenes. It wasn't one person magically appearing here and there, it was two people all along - one wearing a mask and the other with facial scarring that, in a quick view from a distance, looked like a mask. That's my theory, anyway. No ghosts - just two crazy girls.