【Assassin】 , 【Cutthroat】 , 【Gunman】 and 【Bravo】 designate a murderer or one who can be hired to murder in cold blood.

【Assassin】 stresses secrecy and treachery in operation.

• like assassins, these destructive animals do their work in the dark

It is chiefly applied to murderers of important personages.

• tyrants always live in dread of assassins
• Revolutions breed assassins

【Cutthroat】 and 【gunman】 usually designate professional hired murderers.

【Cutthroat】 is chiefly literary or merely figurative because daggers and knives are no longer the weapons usually employed by such criminals, but the word still commonly suggests brutal methods of murder.

• I am a soldier, sir, and not a 【cutthroat】
  —Froude

【Gunman】 is used somewhat more broadly than the foregoing terms since it may denote not only one who murders with a firearm but one (as a gangster) who goes armed and is prepared to shoot to prevent interference with his criminal activities or at the orders of a leader or employer.

Basically a 【bravo】 is a blustering unscrupulous ruffian or desperado.

• a few halfhearted catcalls from young bravos of the opposing party
  —Barker

The word is especially applicable in an historical situation and commonly implies a venality sufficient to perform murder for hire.

• unfolds all of seventeenth-century Italy and its dramas—its predatory noblemen, its murderous bravos
  —Rob
• the hired bravos who defend the tyrant’s throne
  -Shelley