hatchet man 
noun informal
1A person employed to carry out controversial or disagreeable tasks, such as the dismissal of a number of people from employment: he sent over his ace hatchet man to intimidate the business leaders...
- A cafe owner vouches for them and they are freed, but it isn't long before they come face to face with the bloodthirsty robber and his hatchet man!
- Government hatchet men raided Gilani's Delhi home and ‘stumbled’ on a computer file about Indian troop deployments in Kashmir.
- Many of them have been behaving like servile hatchet men and not as members of elite services owing unshakeable allegiance to the Constitution, the laws of the land and the principles of democratic governance.
1.1A person who writes fierce attacks on others or their work: he has abandoned the pretence that he is an impartial reporter—he is a hatchet man of the far right...
- It's not surprising that Krugman is reading a book written by an admitted hatchet man and liar.
- His reputation as the party's hatchet man related especially to his attacks on Adlai Stevenson, the Democrats' presidential candidate in 1952 and 1956.
- The hagiographers and the hatchet men have only begun to do their best - or their worst - on the already weathered marble bust of Rudolf Nureyev's reputation.
Origin
Late 19th century (originally US): figuratively, from an early use denoting a hired Chinese assassin.