A devil is profoundly bad spirit, being, or monster. Your religious uncle might blame all bad behavior on the influence of the devil.
In Christianity, the devil (which is sometimes capitalized), is the essence of evil, also known as Satan. The word is also used to mean a very hateful or cruel human: "He must be a devil, to hurt a kitten like that," or sometimes a mischievous trickster: "What a little devil she is, always playing practical jokes." When you devil someone, you annoy or bother them: "We loved to devil the cranky old man next door."