assassin

(noun) a murderer (especially one who kills a prominent political figure) who kills by a surprise attack and often is hired to do the deed. Synonyms: assassinator, bravo. Examples: "His assassins were hunted down like animals." "Assassinators of kings and emperors."

(noun) a member of a secret order of Muslims (founded in the 12th century) who terrorized and killed Christian Crusaders.