5. Dr. Henry Howard Holmes
In 1893, to celebrate the 400th anniversary of Christopher Columbus' introduction into the New World, the storied Chicago World's Fair took place in the Windy City from May through October and attracted over 20 million people. The most unfortunate visitors checked into a nearby hotel owned and operated by Dr. Henry Howard Holmes, but little did the lucky ones know that the hotel's proprietor was also a ruthless, sadistic mass murderer who'd often kill his guests.
Later called Holmes' "Murder Castle," the hotel became ground zero for the doctor's evilest impulses. Not limiting his murders to guests, he forced many of his female employees to take out life insurance policies in order to start working, would kill them soon after, and then reap the spoils since he was always the beneficiary.
Holmes' approach to murder was, in the most macabre sense of the word, creative. Sometimes, he would lock victims in soundproof rooms full of gas lines and slowly asphyxiate them; for other people, he'd choose the bank vault next to his office, lock them inside, and make them suffocate. A secret chute would send bodies to the basement, where Holmes would remove their flesh, turn them into skull-and-bone models, and sell the skeletons to local medical schools.