It is said that devils feed on negative emotions. I feel this is correct.
But what is a "negative" emotion? Sounds like moralizing, and morality isn't real.
At first, you might at least say they're unpleasant emotions, but Siddhartha was right: they aren't inherently unpleasant, that's something you project onto them. Folk voluntarily seek out fear by watching horror movies or by reading a newspaper - it's not inherently "negative," and likewise I personally enjoy getting angry.
The "negativity" comes from the perception of being damaged, but in the majority of cases it is mere perception. It's not the emotion that's negative, but the injury. Failure is indeed inherently negative. Being weak is indeed a sin.
On the contrary, the emotion is there to help you avoid being injured. Simplifying for brevity, fear informs you of hazards. Anger informs you of treachery. Guilt informs you of your own will to treachery. Your emotions are on your side.
You are what you eat, or perhaps it's more accurate to say you eat what you are. Devils feed on negative emotion because they are formed of negative emotion.
There is one inherently wrong feeling: self-hatred.
E.g. saying emotions, which are on your side, are "negative." Stuff that's on your side is bad? That's a self-hatred.
Self-hatred is self-negating. If you hate yourself, one of the things you hate is the self-hatred. You believe you yourself are wrong, which means you think the self-hatred is wrong, and thus you believe you shouldn't hate yourself. Inherently insane.
Devils are inherently avatars of self-hatred. This is why they are weak. As expected, they are inherently sinful. For them, to renounce weakness is to renounce existing. The Bible claims any idiot can rebuff a devil, and this is correct. They embrace failure and suffering; if you do not likewise embrace their failure and suffering, you give them nothing to work with. If instead you validate their self-condemnation, then they are condemned. "I suck." "I agree, you suck." Game over.
True devils are extremely rare. They are self-destructive, so their masses cannot accumulate over time. For the most part if you don't intentionally seek them out you will not encounter them at all. Even rebuffing them is unpleasant; why do it if it's also unnecessary?
Hatred is a thing that exists, and selves are a possible thing it could be directed towards, and I suspect this is related to the reason devils have to exist in the first place.