Writer's Fallout Shelter

Actually, it is a fairly modern thing that fairies, elves, pixies, and other such spirits are portrayed as "good." It seems like we forget the creatures themselves, and remember only their beauty. Faries were once things to be feared. They'd lure you into their realms where you wouldn't be able to escape for the rest of your life. They'd also be terrific tricksters (i.e. A Midsummer Night's Dream).

It is also possible that such stories arise from ancient peoples' misunderstanding about the world around them. For example, some of the first vampires weren't vampires as we know them today. The corpse would be in the ground, but it's "spirit" (for lack of a better word) would roam the night in search of prey. (Note: there are many different variations on this.) When people exhumed the corpse they would note that it had grown bloated, as if from the blood of others, and it would remain loose instead of developing the stiffness of corpses. When they stuck a stake in the heart then it would let out a groan, as if from its last breath. It would also bleed. Anthropologists today, however, find that all these things are perfectly explainable by the natural decay process. The body often traps gasses, for example, hence the 'groan' when pierced through the heart. It is possible that the vampire myth simply sprung up from insufficient understanding of what the body does after death.