There are some very strange groups in history.
How about the Indian thuggees? Their cult is the source of our English word thug:
Thuggee or tuggee (Hindi: ठग्गी ṭhagī ; Urdu: ٹھگ ; Sanskrit: sthaga ; Sindhi: ٺوڳي، ٺڳ ; Kannada: "thakka" ) refers to the acts of Thugs, an organized gang of professional assassins.
The Thugs travelled in groups across India for six hundred years. Although the Thugs traced their origin to seven Muslim tribes, Hindus appear to have been associated with them at an early period.
They were first mentioned in Ẓiyāʾ-ud-Dīn Baranī's History of Fīrūz Shāh dated around 1356. ... The Thugs would join travelers and gain their confidence. This would allow them to then surprise and strangle their victims by pulling a handkerchief or noose tight around their necks. They would then rob their victims of valuables and bury their bodies. This led them to also be called Phansigar (English: using a noose), a term more commonly used in southern India.
Thuggees worshipping (grin)
The earliest currently known recorded mention of the Thugs as a special band or fraternity, rather than as ordinary thieves, is found in the following passage of Ziau-d din Barni's History of Firoz Shah (written about 1356):
In the reign of that sultan (about 1290), some Thugs were taken in Delhi, and a man belonging to that fraternity was the means of about a thousand being captured. But not one of these did the sultan have killed. He gave orders for them to be put into boats and to be conveyed into the lower country, to the neighbourhood of Lakhnauti, where they were to be set free. The Thugs would thus have to dwell about Lakhnauti and would not trouble the neighbourhood of Delhi any more.
—Sir HM Elliot, History of India, iii. 141.
Membership was sometimes passed from father to son, in what would now be termed a criminal underclass. The leaders of long-established Thug groups tended to come from these hereditary lines, as the gang developed into a criminal 'tribe'. Other men would get to know a Thug band and would hope to be recruited, in the way that one might aspire to join an elite regiment or university: they were the best operators in "the business" and, like a regiment or college fraternity, once in the group, there was a camaraderie of numbers and shared experience. The robbery became less a question of solving problems of poverty and more a profession, like soldiering.
Sometimes the young children of the travellers would be spared and groomed to become Thugs themselves.
reference: http://en.wikipedia.org/wiki/Thuggee