But contrary to most popular belief that the Doll killed everyone, I think on more of the lines that the Doll symbolizes Aika. ._.

The first house seems perfect and happy, but with a closed back room... As if the house is hiding secrets.

The second house has full of exit signs and dolls sitting in the basement, perhaps Aika found out something so terrifying that she didn't want to think about it and just run away? Because, interestingly, the back room has dolls facing away, as if not wanting to face reality. The basement with a doll party is probably her desire to be have a happy and whole family, and yet the top has a rather symbolic Garden of Eve style. I read from somewhere that the male statue is actually real, and the female statue is actually fake. So, perhaps Aika's mother must have committed a 'sin'? The graveyard and the dogs... maybe it's the death of innocent...?

Aika loves her mother very much. Her town flag is her mum, her default statement declares her love for her mum. Perhaps, to her, her mother was perfection... but what if her mother wasn't as perfect as she thought? And what if that realization snaps her? At the third house, the right room has a piano and egg shells, which could be interpreted in various ways. Most 'ideal', 'perfect' children plays an instrument. So, maybe Aika was under a lot of pressure to be 'perfect'? Perhaps it was something she accepted only cause she thought her parents, her mum is 'perfect'? On the top room there's a sketch book and a 4 leaf clover. We can see in the first house that Aika loves drawing, drawing her 'perfect and happy family'. What if the revelation that nothing is imperfect in this world snapped her, hence the pages all around? o.o And maybe, just maybe, because her mother was 'imperfect' in this 'perfect' home, Aika decided to axe her out? Hence the underground murder symbolism...?

At the last house though, it was back to the layout of the first house with trash scattered about. The upper room has the Doll and Aika mannequin isolated in the middle of the room. Perhaps after doing the deed, Aika locked herself away mentally and emotionally? The two items could symbolize both of her facets: the mannequin may be her naivety, the doll was her desire for perfection? On top of the room, her mother was blotted out in one sketch, and Aika was blotted out in another. Maybe Aika thought her mother wasn't perfect, and now having done the 'deed', she wasn't perfect anymore? Perhaps in the end, Aika felt remorse and bury her mother and 'perfection' (or belief of perfection) together... and then committed suicide?

That's my theory anyways.