evil genius

Noun

evil genius (plural evil geniuses or evil genii)

1. The spirit each person is believed to have in attendance, according to certain religious or mythological traditions, which tries to negatively influence him, and is opposed by his good genius; loosely, someone who is a bad influence.
   • He could in nowise comprehend it; but, taking it with other previous circumstances, he could not get quit of a conviction that he was haunted by some evil genius in the shape of his brother […].1824, James Hogg, The Private Memoirs and Confessions of a Justified Sinner, Oxford, published 2010, page 37
   • It was as if I unconsciously realized that in her I had met my evil genius.1957, Lawrence Durrell, Justine
2. An intellectually brilliant person who excells at using his or her mental abilities for negative or harmful ends; especially a criminal mastermind.