Assassination is the act of murdering an important individual or individuals, usually political figures. In many cases, ideological or politics provides the primary motivation for assassination. Other motivations include financial gain, revenge, or for military reasons. Assassination has played a major role in many different cultures throughout galactic history.
On Earth, assassination has played a major role in human history. The early Roman leader Julius Caesar was assassinated in 44 BC, and this act was later the subject of a play by William Shakespeare. Later, several Presidents of the United States of America were assassinated while in office. The most notable of these were Abraham Lincoln and John F. Kennedy. (TOS episodes: "The Cage", "The Savage Curtain")
Indira Gandhi, who served as Prime Minister of India on two occasions between 1966 and 1984, was assassinated by two of her bodyguards on October 31, 1984. She was succeeded as Prime Minister by her eldest son Rajiv Gandhi, who was himself assassinated on May 21, 1991, two years after leaving office. (TOS novel: The Rise and Fall of Khan Noonien Singh, Volume 1, The Rise and Fall of Khan Noonien Singh, Volume 2)
In 2293, Chancellor Gorkon was assassinated by a conspiracy led by the Klingon General Chang, the Romulan Nanclus, and Federation officials Admiral Lance Cartwright and Colonel Patrick West. Captain James T. Kirk and Doctor Leonard McCoy were initially accused of and found guilty of this assassination before the true assassins were exposed. The crew of the USS Enterprise-A played a key role in exposing this conspiracy, and preventing the assassination of the Federation President Ra-ghoratreii. (TOS movie: Star Trek VI: The Undiscovered Country)
The members of the Romulan Senate were assassinated by Senator Tal'Aura on the behalf of Shinzon of Remus in 2379. As a result, Shinzon briefly took over the Romulan government until he was killed in battle with the USS Enterprise-E. This assassination had profound consequences for the Romulan Star Empire. (TNG movie: Star Trek Nemesis)
During the Occupation of Bajor, the Bajoran Resistance targeted high ranking Cardassians for assassination. A number of attempts on the life of Gul Skrain Dukat were made during his time as Prefect. In 2369, the Bajoran Resistance group Kohn-Ma claimed responsibility for the assassination the First Minister of the Republic of Bajor. (DS9 episodes: "Things Past", "Past Prologue")
In the mirror universe, assassination was a common method of advancement in the Imperial Starfleet. Captain Christopher Pike assassinated Karl Franz to become the commanding officer of the ISS Enterprise, and was in turn later assassinated himself by James T. Kirk. When the James T. Kirk from the primary universe visited the mirror universe in 2267, he was very nearly assassinated by Pavel Chekov, who believed him to be his counterpart. (TOS episode: "Mirror, Mirror")
In the Star Trek: Mirror Universe continuity, the mirror Kirk was in turn assassinated by the mirror Spock shortly after his return from the primary universe. (TOS novel: The Sorrows of Empire) However, in ST novel: Spectre, Spock, aware that assassinating Kirk would lead to his own death a short time later, did not kill the mirror Kirk.