Some dragons are good. Not all dragons.
The village was burning as the black dragon circled overhead. The knight pointed his crossbow and fired another bolt. It missed again, deflected by the dark magic protecting the dragon. He swore and reloaded, but he was too late - the dragon was swooping towards him. He was roasted alive in his armour before he could scream.
The black dragon was hungry, and he had been searching for days before finding the village. The humans were foul tasting and the sheep had little meat with wool that stuck between his teeth, but they would do. This dragon had come to the Earth from the land of Ender, and it knew not how to get home. It finished off the last of the villagers and flapped into the air, seeking out more of the humans. It amused him to see them dashing about and panicking before being devoured, even if they did taste awful and barely filled his belly.