Assassin

An assassin (暗殺者) is a person who is trained to kill other people, normally for political reasons or personnel reasons.

Assassins in One Piece
The Baroque Works agents are known to have assassins in Whiskey Peak. Mr. 5 and Miss Valentine tried to kill Princess Vivi but failed. Mr. 3 tried to kill the Straw Hat Pirates, Vivi, Dorry and Brogy.

Nico Robin claimed that she is the assassin but judging by her past story, it was unknown who she assassinated. However she did try to kill Crocodile because he was going to kill her.

The CP9 are the first assassins in the series to be from the World Government and they are the only World Government assassins. Lucci, Kalifa, Blueno, and Kaku were assigned to Water 7 to get the blueprint and kill Iceburg. Meanwhile, Jyabura, Fukurou and Kumadori were sent to an unnamed town to assassinate three Revolutionaries key leaders (Dragon not included). However, they ended up killing 20 more thanks to Fukurou. Among the Straw Hat Pirates, Robin might be the assassin since she has high assassination skill.