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.
    • 1824, James Hogg, The Private Memoirs and Confessions of a Justified Sinner, Oxford 2010, p. 37:
      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 […].
    • 1957, Lawrence Durrell, Justine:
      It was as if I unconsciously realized that in her I had met my evil genius.
  2. Literally, a brilliant person who uses his abilities for negative or harmful ends; especially a criminal mastermind.