THUG - which is defined by the Mirriam Webster Online Dictionary as, "a brutal ruffian or assassin : GANGSTER, KILLER" comes from the name of a 7th century Indian religious cult that terrorized India until the mid 19th Century. The Thuggees engaged in ritual murder to serve the Hindu goddess Kali (remember Indiana Jones and the Temple of Doom?). From Hoffman:
On specified holy days throughout the year, group members would forsake their daily occupations and lie in wait for innocent travellers who would be ritually strangled as sacrificial offerings to Kali. According to some accounts, the Thugs killed as many as a million persons during their twelve-hundred-year existence, or more than 800 individuals every year: a murder rate rarely achieved by their modern-day counterparts armed with far more efficient and destructively lethal weaponry.
Assassin - defined generally as "one who murders a politically important person either for hire or from fanatical motives", originally used to be the name of a secret order of Muslims that at the time of the Crusades terrorized Christians and other enemies by secret murder committed under the influence of hashish (which is the origin of the word: hashshAsh one who smokes or chews hashish).
Finally, the word Zealot - which now refers to a zealous person; especially : a fanatical partisan; used to be the name of a member of a fanatical Jewish sect arising in Judea during the first century A.D. and militantly opposing the Roman domination of Palestine.
There is a lot of fascinating history in Hoffman's book and--although it was written before the events of 9/11--it is a fascinating summary of some of the historical trends underlying international terrorism. Highly recommended.