Uzumaki's horror is a brute fact — the origin question is malformed
The clearest demonstration that Ito isn't doing J-horror: the antagonist of his most famous work is a shape. The town of Kurouzu-cho becomes infected by the concept of the spiral. Pottery, hair, smoke, snail shells, ear canals, fingerprints, the curl of a tongue, the structure of the cochlea, the path of bodies in death — everything containing a spiral begins to be more spiral, and people warp accordingly.
There is no wronged spirit here. No cosmic debt. The spiral doesn't want anything and isn't the ghost of anything. It is a Platonic form that has acquired hostile autonomy. The horror is abstract and indifferent.
The onryō tradition has nowhere to put this, because the onryō tradition is fundamentally relational. The cinematic ghost story is structured around disclosure — you find out who Sadako was — or its deliberate refusal, as in Kurosawa's films where the origin never comes. Either way, the question of origin is the spine of the narrative.
Uzumaki doesn't owe you an explanation of where the spiral came from, because the question is malformed in the world of the manga. The spiral is. Asking "but why a spiral" is like asking why π is irrational. Ito's horror is the horror of a brute fact.
This is also why Uzumaki adapts badly to live action. The cinematic onryō grammar — long static shots, ma, the wet-haired figure — is built for an entity that wants something from you. A spiral doesn't want anything. There is no relational frame for the camera to hold.
Source claim: Uzumaki's horror is a brute fact with no origin to disclose — the question "why a spiral" is malformed, which makes it metaphysically incompatible with the onryō tradition's relational spine.