TBH these voices might have been the original reason why "demons" got defined by the ancients as evil spirits.

They often speak evil things, are unseen to the general population, but torment the sufferer ... a basic candidate for the category of "demon", especially in pre-modern times.

Also, I don't think that demons are a particularly Christian phenomenon. They certainly predate Christianity and various "heathen" religions were full of them: Indo-European, Amerindian, African...

But the idea of schizophrenia as a very intense mind going on empty is interesting. Only I would say that it needs some specific pathway to go down the schizophrenic route. Plenty of people with intense fantasies are normally adjusted and don't fulfill the criteria for a mental disease.