The Grudge/WMG

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 KULT
People 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.