hatchet man

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

Company employee responsible for reducing personnel or telling them of their dismissal and the terms of severance.

1.  A person assigned or hired to carry out a disagreeable task or unscrupulous order. For example, When it came to firing an employee, Arthur was his boss's hatchet man. This expression originally referred to a hired assassin but in the mid-1900s was transferred to less nefarious enterprises.
2.  A person who attacks the reputation of others, especially a journalist hired to do so, as in You can count on Mary's column to destroy the mayor--she's the perfect hatchet man. This usage gave rise to hatchet job, meaning "harsh destructive criticism." [Mid-1900s]

The noun has 2 meanings:

Meaning #1: a professional killer
  Synonym: iceman

Meaning #2: one whose job it is to execute unpleasant tasks for a superior
  Synonym: enforcer