hatchet man

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

• noun A man hired to commit murder.
• noun One assigned to carry out a ruthless task or a vicious or unscrupulous order.

from The Century Dictionary.

• noun One who wields a hatchet, in any kind of work.

from the GNU version of the Collaborative International Dictionary of English.

• noun A person hired to murder or physically attack another; a hit man.
• noun A person who deliberately tries to ruin the reputation of another, often unscrupulously, by slander or other malicious communication, often with a political motive, and sometimes for pay.

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

• noun A professional killer.
• noun colloquial, idiomatic Someone who carries out brutal and unpleasant duties on behalf of another, such as firing dead wood employees.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun a professional killer
• noun one whose job it is to execute unpleasant tasks for a superior