In shinto everything is spiritual and therefore there is a continuity between humanity, nature, and the spirits/gods. The continuity between humanity and spirits is depicted in the movie because the demon was once a human, but turned into a demon.
Also, fox as a kami of agriculture can have up to nine tails, and more tails implies more power. Power is usually symbolized by the quantity/number of some sort of body feature/part. This agrees with the scene that the demon said “even here we have grades, one-horned demons like myself always get eaten by those with two or three horns” (Dreams-The Weeping Demon Pt. 2).
Moreover, one of the general characteristics of a life lived in reverence of Kami (the deities) is to be helpful to others and in the world at large through deeds of service without thought of rewards, and to seek the advancement of the world as one whose life mediates the will of Kami.
The demon was a farmer when he was a human, but because of his thoughts of rewards (money/high price for his produce), he used to bury food. By doing this he did not serve the world without thought of reward and did not live a life in reverence of kami. This could be part of the reason he (or the demons) could not get food but had to feed on each other by killing themselves.
