Buddhism actually does have a concept of evil as personified by the demon Mara who represents sin, evil, and temptation. After all, Buddhism pretty much started as a way to correct the evils of Brahmanism, which means that there was a concept of wrong/evil being revolutionized by good/right thinking. "Pure" Buddhism, however one may define that, holds Mara as the only evil force in the word, but as Buddhism spread throughout Asia and mingled with the traditional folklore of the different peoples across the continent, Buddhist mythology became populated with little evil spirits of all kinds that represented the ills of the world. This is especially true for the forms of Buddhism that developed in China and Japan. Unfortunately, I haven't studied Buddhism enough to say whether or not demonic possession plays a role, whatever the size, in Buddhist belief (my focus in my Religion major was Early Christian History. If you have any questions about that, I'm your gal!).
