If you refer to someone as a robber baron, you mean that they have made a very large amount of money and have been preparedto act illegally or in an immoral way in order to do so.
...the vast wealth accumulated by America's robber barons from industry and transport.
robber baron in British English
(ˈrɒbə ˈbærən)
noun
a person who has made a very large amount of money and has been prepared to act illegally or in an immoral way in order to do so
robber baron in American English
1.
a nobleman of feudal times who robbed people traveling through his domain
2. US
any of a number of U.S. capitalists of the late 19th cent. who acquired vast wealth by exploitation and ruthlessness
Examples of 'robber baron' in a sentence
robber baron
With robber barons in charge, incompetence and irresponsibility is becoming the norm.
Times, Sunday Times (2015)
Many critics assert that modern managers have no more social conscience or "soul" than the nineteenth-century robber barons.
Hunt, E. K. Property and Prophets: The Evolution of Economic Institutions and Ideologies (1995)