Faeries come from the dreams of children. Each faerie is dreamed into life by a particular child, or the way that old, wise faeries tell it, they dreamed themselves to life.
Each faerie inherits a few traits from their dreaming child: language, regional knowledge, favorite foods, nursery rhymes, etc.
And because they come from children's dreams, faeries are wildly varied and inherently chaotic. The capriciousness of faeries is the capriciousness of children. The whimsy of faeries is the whimsy of children. The cruelty of faeries is the cruelty of children.
That's why faeries kidnap children. They take them away and put them to sleep. (They put them in diabetic comas by feeding them candy.)
Faeries prefer to approach children (and childish adults). Though they give kisses, they have no concept of sex. And although they kill, they do not understand death.
Faerieland is filled with sleeping children. Great mounds of them. Orphanages where they are stacked like firewood.
Some children are trapped in nightmares, and terrible things come out of their heads. But that's okay. Faeries can't even spell 'consequences'.
And if one of those children is woken up, somewhere a faerie dies forever. They don't disappear; they suffocate. Eyes rolled back, gasping, as nightmares curl around their heads.