They operate under Blue-and-Orange Morality, and neither understand nor care about human morals.

They are or have been persecuted by mortals and fear them as much as humans fear them. They think themselves fighting to survive against inscrutable, dangerous creatures armed with iron — and perhaps are to some degree.

The Fair Folk are obsessed with political disputes between the rulers of Otherworld. What seems to us erratic behavior is in fact the collateral effects of their elaborate schemes.

There are criminals in Otherworld just as in the human one. The Seelie are the respectable fey and the Unseelie are the criminals. Naturally they would behave badly in the mortal world; they behaved badly in Otherworld too. Which means, several of them have a bounty on them from the King of Otherworld. If you can catch one, you will be granted riches untold, a bountiful harvest, or whatever.

The Fair Folk cause chaos and kidnap children because its part of their religious traditions. Their god is an Eldritch Abomination that feeds on chaos, which is why they cause mayhem and torment in the mortal world, and their god also demands that they sacrifice human children to it on a regular basis.

The Fair Folk come from a dimension that has completely different rules than the mortal world and does not make sense to us. Why would the fairies themselves be any different?

The Fair Folk are Obliviously Evil; what humans see as mayhem and torment, the Fair Folk genuinely see as playful behaviour. They kidnap children because they just want some friends to play with.

Fairies not only have Blue-and-Orange Morality—their allergies to Cold Iron and salt, plus their inherent magical abilities, is a sign that they're not QUITE the same species as humans.