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.
Literally, a brilliant person who uses his abilities for negative or harmful ends; especially a criminalmastermind.