a criminal, often very bright and very dangerous when he or she is loose in the organization

Is a person who operates outside the limits of his authority. There may or may not be criminal intent. Often these rogues try to hide losses but in the process incur even greater ones. Often these traders take on larger positions which are not authorized in order to recoup open losses.

Rogue Trader: How I Brought Down Barings Bank and Shook the Financial World is a book by Nick Leeson, who served four years in prison for fraud after bankrupting the London-based Barings Bank in 1995 by hiding $1.4 billion in debt he accumulated as a derivatives trader in Singapore.