7. Fritz Haarmann
Fritz Haarmann was one of the first serial killers of the modern era. Between 1918 and 1924, he admitted to killing between 50 and 70 young men, although police could only definitely link him to 24. His arrest created a media circus in his native Germany, where he was referred to as the Vampire of Hanover, due to his tendency to bite the necks of his victims. He and his lover/accomplice Hans Grans sold many of their victims' personal items on the black market to make ends meet. It's even rumored that, as a contraband butcher, he sold meat from his unlucky victims. Interestingly enough, Haarmann was a well-known police informant. Having lived as a con artist and petty thief for many years, he was familiar with the criminal underground and was willing to provide information to the police for a price. Throughout their entire investigation, they never realized that their culprit was someone who worked within the police infrastructure.