hatchet man

hatchet man  {n.},  {colloquial}

1. A politician or newspaper columnist whose job is to write and say unfavorable things about the opposition.

Bill Lerner is the hatchet man for the Mayor's Party; he smears all the other candidates regularly.

2. An executive officer in a firm whose job it is to fire superfluous personnel, cut back on the budget, etc., in short, to do the necessary but unpleasant things.