[note 1] which was never intended to be taken seriously, since it isn’t possible to mathematically prove anything that is not part of mathematics

Now, I don’t like this answer, since it says what IS greater than God, what IS more evil than the Devil, etc.
Note the word IS. Nothing (as a concept) can’t be greater than something, since nothing, by definition, ISN’T.
“Nothing” can’t be more evil than something evil, since “nothing” can be considered the baseline. In Christianity, the baseline is that the universe is good, and evil arises from misuse of free will; since “nothing” isn’t a being, then it can’t have free will, therefore can’t be evil. In many other religions, neutral (neither good nor evil) is the baseline, and it is a choice to be good, and a choice to be evil. “Nothing” can’t have made a choice, since “nothing” isn’t a being; therefore it isn’t evil.
The bit about the poor having nothing and the rich needing nothing sort of makes sense, except that it is using the word in different ways. The rich need nothing *more* than they have, but if they suddenly had *nothing*…

Anyway. I haven’t read all the comments, apart from that there appears to be a lot of arguments about religion/belief, which, though important to the majority of the population of the world (yes, atheism is just as much a belief without proof as religion, since there can be no proof that a god does *not* exist. After all, the world (and science) is so complex that if a god exists they don’t have to break or bend the laws of physics to intervene. Particularly with quantum. Anyway, rant over)… where was I… oh yes. Religion, though important to the majority of the population of the world, is not important to this riddle. I first heard it in a D&D sourcebook (can’t remember which one) as something for a sphinx to say, in which it was “what is more powerful than the gods, more terrifying than the demons, the poor have it, the rich need it, and if you eat it, you will die”, and no-one irl (don’t quote me on this) believes in the gods of D&D.

What I was going to say was that my answer would be Fear.
1. From a Christian standpoint, God will never go against your choice. You have free will, and God respects your choices. If you are fearful, then fear will prevent you from accepting God’s intervention.
2. The Devil only has power over you because of fear. Therefore, fear grants the devil his powers, therefore, fear is more evil than the devil, just like Hitler is more evil than the people who obey his orders.
3. The poor are always fearful, since they have no security.
4. The rich need fear (e.g. of the law, of God/the gods, etc.) to prevent them from exploiting their power in a way that is damaging to others.
5. If you feed on fears, then your fears become so great that you die, whether literally (suicide or similar) or metaphorically (mental breakdown etc., where you loose your identity but remain alive in body)

Twos-day (Tuesday)