We grew up on fairy tales wherein fairies would come to the aid of a young and deserving hero or heroine. They’re benevolent. And all they want to do is to help
But once you go beyond the children’s section of the bookstore, you’ll find that fairies or the fae are usually depicted as cruel or mad or simply out of touch with normal human reality and morality. Being near-immortal and powerful has them usually portrayed as looking down on humans (though that doesn’t stop them from taking humans as pets/lovers and producing half-human offspring with them)
Of course, this is not to say all fae characters in books are evil. Some harm humans as a way to protect themselves. Fae are usually portrayed as very connected to nature (eg. forests, lands, rivers etc). And with human beings doing whatever they want to the land, it seriously pisses the Fae off that their home gets polluted and eventually destroyed.
This just goes to show that once you browse the sections for young adults and adults, Fae get the morally gray and/or black shade