The city with the worst recorded homicide epidemic in the 'world' - not the United States. :/
As far as history and legend goes, Medellin in Colombia. A city decimated by murderous street gangs, power-crazed drug cartels and guerilla/paramilitary bloodletting.
Throughout the 90's and early this century, Medellin held the title, peaking with around 400 homicides per 100,000 in 1991. This fell to about 150 per 100,000 during the mid to late 90's but rose again in the early 00's, approaching 200 per 100,000. These figures are even more remarkable considering Medellin is a decent sized city of 2 to 3 million people within the period covered.
Thanks to a phenomenal security clampdown Medellin is no longer the 'murder capital of the world', but it remains the ultimate example of what can happen when crime literally spins out of control.