Demon

(noun) an evil supernatural being. Synonyms: devil, fiend, demon, daemon, daimon.

(noun) a cruel wicked and inhuman person. Synonyms: monster, fiend, devil, demon, ogre.

(noun) someone extremely diligent or skillful. Synonyms: demon. Examples: "He worked like a demon to finish the job on time." "She's a demon at math."