Answer:
Shakespeare's play "MacBeth" is the earliest known work of literature to contain the word "assassination," but it's probable that he did not actually invent the term itself.
Assassination is derived from an older Arabic term "hashashin," which means someone who murders for a certain reason or end. There is some evidence that this, and other Arabic words adopted into English, were brought back to Britain from as early as The Crusades, or later as trade through the Middle East blossomed during The Renaissance.
Also bear in mind that much of literature before Shakespeare's time has not survived, so while MacBeth may be the earliest use we know of in print, it may not have been the first time assassination was actually written.