I guess if we are going with real to life individuals, and ASSUMING the tales are true, here are a couple you would NOT want to risk dealing with, IMO.
Herman Webster Mudget - You might know him as H.H. Holmes. He was the first person classified as a "serial killer" and he was one sick son of a bitch. Unlike a lot of modern "pussified" serial killers that just attack much younger and lighter women, this guy did a lot of bad stuff to everybody he came across. He was convicted of killing 4, confessed to 27, but was highly suspected in the murder and complete disappearance in as many as 200 people.
Frank Dux - The inspiration for Jean-Claude's Bloodsport. If the stories can be believed, the guy was a martial artist of amazing ability (broke two magnum champagne bottles with one spinning back kick, punched through "bullet proof" glass) and he went to work for the CIA doing wet work operations directly for Bill Casey. Many claims have been made that he was a fake, but if he was not, he was one bad ass dude.
Richard "The Iceman" Kuklinski - Look him up. If his story is to be believed, he was the most prolific hit man and serial killer in the history of the term "hit man".
Roy Albert Demeo - headed the only known group of cooperating serial killers for the Gambino crime family. A gang that included Kuklinski. They killed so many people, and disposed of so many bodies, police and FBI do not have a reasonable estimate of how many people they killed.
Now those are some scary mo-fos!