A crime syndicate doesn’t have to be huge to make this list – it just needs to be terrifying. For residents of the British town of Islington, the Clerkenwell Syndicate is enough to make you lock your doors. Founded in the 1980s by the three Adams brothers, the gang quickly grew notorious for their willingness to get their hands dirty. Youngest brother Patrick Adams was notorious for his violent nature, employing a trademark method of speeding up to targets on a motorcycle, unloading a pistol into them and tearing away before anybody realized what was happening. The group has been linked positively to more than 25 murders and are suspected of dozens more. One of their prime sources of money is, of course, extortion, with just about every business in Islington lining their pockets under fear of death.