Assassin - One who murders by surprise attack, especially one who carries out a plot to kill a prominent person.
From "Go Postal" to "assassin", I seem to be getting away with murder. Stop me, someone, before I go shooting from the hip. Awlright, enough of pretentious play of words, it's time for some history lessons, now :)
During the time of the Crusades, the members of a certain secret Muslim sect terrorized their enemies by performing murders as a religious duty. Active in Persia and Syria from the 8th to 14th centuries, the original Assassins were members of the Nizaris, a Muslim group who opposed the Abbasid caliphate with threats of sudden assassination by their secret agents. Other populations of the area regarded the Nizaris as unorthodox outcasts. Because these acts were supposedly carried out under the influence of hashish, the killers came to be known as hashshashin - eaters or smokers of hashish. It appeared in its anglicized form in the 17th century.
Trivia: Marco Polo tells a tale of how young Assassins were given a potion and made to yearn for paradise—their reward for dying in action—by being given a life of pleasure.
* End of History class*
Sources: The Merriam-Webster Book of Word Histories, http://www.answers.com/