The cat is behind everything.Almost every death caused by the poltergeists involved the little ghoul, Toshio, meowing like a cat. Although one might think this reflects his obsession with his dead cat, it's the other way around. The human spirits have all moved on, and it is the ghost of the cat alone striking out with its feline fury. The cat was a bakeneko — a shapeshifting, human-eating ghoul cat of Japanese myth. It was killed by the family's father, which made it a powerful demonic entity. It hunts down victims in the various shapes of its former owners and then devours them, explaining the conspicuous lack of bodies. It explains, among other things, why these ghosts are preternaturally powerful. It's all the cat.
The final film in the series will star Hellboy.Honestly, this sort of thing is his stock-in-trade - and who else would be able to send the ghosts off with a bang?
The whole backstory is a lie. The spirits of the family are actually demons from Hell.Really? I mean, you and your family is murdered by your husband, and THIS is your response? Kill random, unconnected people? What the hell, revenants? Obviously the backstory is one giant Red Herring to keep the victims from finding a good Catholic Priest.
The curse is sentient.The curse came to sentience when it was born , desiring only to reproduce. People killed by the ghosts are permanently absorbed, becoming nothing more than puppets for the curse to perpetuate itself.
The house is a portal to Inferno of KULTPeople killed within the house solidifies it's presence in Inferno, allowing more of creatures through. Humans killed here become trapped between the fabric of the illusion and Inferno (ie. The Ghosts). The entire project is being overseen by an Angel of Death in order to make more people believe in the Inferno, and thus, end up there.
The curse is a ghostly blood splatter.Okay, so you know how if you got crushed by something big enough, not even a single recognizable remnant of your corpse would remain? You'd just be like, splat. And then liquidy gory stuff would be everywhere. You know how also Kayako died "in the grip of a powerful rage?" What if the rage was so powerful that it crushed her soul in its grip? No recognizable traces of her mind or emotions left in the resulting mess. Just, splat. Ectoplasm everywhere. That's why her ghost almost never says anything, or has any discernible motives. That's why it sticks around in one place, repeats everything she did in life, and kills people indiscriminately. And that's why all her family members' ghosts seem to share some kind of creepy hive mind. Because her soul and her family's souls aren't in one piece anymore. They're just, splat. Strewn all over the walls and floor of the house. Just boiling and mushing together. They're no longer individual spirits, just one indistinct, integrated mess. One with one another. One with the house. One with chaos, disorder, and rage. And that's also why the curse follows people even after they leave the house, and why it swallows them up and uses them after they die. Could you wade through a pool of pureed organs and not get any on you? Didn't think so.