H. H. Holmes. America's first serial killer.

via Wikipedia.

Holmes purchased a lot across from the drugstore, where he built his three-story, block-long "Castle"—as it was dubbed by those in the neighborhood. It was opened as a hotel for the World's Columbian Exposition in 1893, with part of the structure used as commercial space. The ground floor of the Castle contained Holmes's own relocated drugstore and various shops, while the upper two floors contained his personal office and a maze of over 100 windowless rooms with doorways opening to brick walls, oddly-angled hallways, stairways to nowhere, doors openable only from the outside, and a host of other strange and labyrinthine constructions. Holmes repeatedly changed builders during the construction of the Castle, so only he fully understood the design of the house.[3]
During the period of building construction in 1889, Holmes met Benjamin Pitezel, a carpenter with a past of lawbreaking, whom Holmes exploited as a stooge for his criminal schemes. A district attorney later described Pitezel as Holmes's "tool... his creature."[11]
After the completion of the hotel, Holmes selected mostly female victims from among his employees (many of whom were required as a condition of employment to take out life insurance policies, for which Holmes would pay the premiums but was also the beneficiary), as well as his lovers and hotel guests. He tortured and killed them.[8] Some were locked in soundproof bedrooms fitted with gas lines that let him asphyxiate them at any time. Other victims were locked in a huge soundproof bank vault near his office, where they were left to suffocate.[6] The victims' bodies were dropped by secret chute to the basement,[3]where some were meticulously dissected, stripped of flesh, crafted into skeleton models, and then sold to medical schools. Holmes also cremated some of the bodies or placed them in lime pits for destruction. Holmes had two giant furnaces as well as pits of acid, bottles of various poisons, and even a stretching rack.[3] Through the connections he had gained in medical school, he sold skeletons and organs with little difficulty.

I heard an interview on this guy last and night and he's a piece of work.

They estimate that he could have killed up to 200 people.

He built his kill house in Chicago just before the World's Fair.

He lured women to his maze filled house and also killed a few men and at least 3 children.

Lastly, this bastard was so vile that he had a woman placed on a stretching table, impregnated her and continued to stretch her so he could see if that would make a taller race of people.

The documentary,  H. H. Holmes: America's First Serial Killer, is something I'm definitely gonna have to see.  And if this wasn't bad enough, the FBI believes that at any one time there are at least 35 to 50 serial killers active in the US.

A little side dish.  Check out this cracked article on serial killers who are free right now.