Nico Robin claimed that she is an assassin when Usopp interviewed her upon her entry into the Straw Hat crew, but her targets and reasons have yet to be revealed. However, she was prepared to kill Crocodile since he recruited her into Baroque Works, and tried it when he declared that he was going to kill her, though her attempt ultimately failed.
The CP9 are the first assassins in the series to work for the World Government and they are the only known World Government assassins. Lucci, Kalifa, Blueno, and Kaku were assigned to Water 7 to get the Pluton blueprints and to kill Iceburg. Meanwhile, Jabra, Fukuro, and Kumadori were sent to an unnamed town to assassinate three key Revolutionary leaders (Dragon not included). However, they ended up killing twenty extra people thanks to Fukuro revealing his team's presence and their intentions to the town. This strongest generation has been dismissed as criminals when their superior, Spandam, framed them of a debacle at the Enies Lobby.
Hyouzou is a mercenary hired by the New Fishman Pirates to become an assassin of the crew. His profession came to an end when he overdosed on Energy Steroids, which eventually stripped him of all of his natural prowess, rendering him a frail and weak old man, and the fact that he was arrested and put in the prison of the Ryugu Palace.
The term koroshi-ya (殺し屋,koroshi-ya?) technically means "professional killer". The more correct kanji term for "assassin" is ansatsu-sha (暗殺者,ansatsu-sha?) or shikaku (刺客,shikaku?). Although, the three can be used synonymous.