Angels and demons are both manifestations of the human psyche. Good and evil are constantly up for debate, because they are largely matters of perception. And perception happens in the mind. They are manifestations of life - even death and killing, because for death to happen, for murder to happen, there must first be something alive.

So what is the opposite of both an angel and a demon?

Mindlessness. Insanity. Negation. The gnostic Demiurge. The horror of oblivion, of that which destroys with recreating, the thing that breaks the truth of e=mc2.