robber baron

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun One of the American industrial or financial magnates of the late 1800s who became wealthy by unethical means, such as questionable stock-market operations and exploitation of labor.
• noun A feudal lord who robbed travelers passing through his domain.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun historical In Europe, an aristocrat who charged exorbitant fees or otherwise exacted money from people who journeyed across land or waterways which he controlled.
• noun idiomatic, usually derogatory Especially in the 19th-century and early 20th-century, a business tycoon who had great wealth and influence but whose methods were morally questionable.