3. The Spahalski Brothers
Although identical twins Stephen and Robert Bruce Spahalski were both killers, each acted alone, unaware of the other's homicidal tendencies. "I thought I was the only murderer in the family," Stephen reportedly said in 2005 when an Attica, N.Y., corrections officer showed him a newspaper article about his brother confessing to four slayings. Stephen was the first to kill, fatally stabbing a man, but Robert ended more lives, including a prostitute, whom he strangled with an electrical cord, and his girlfriend, whom he choked to death. His brother, Stephen, was a teenager when he pleaded guilty to manslaughter and over the years would return to prison for robbery, kidnapping and parole violations. The brothers' extensive criminal history, detailed in Killer Twins, a 2010 book by true-crime author Michael Benson, continues to fuel the nature-vs.-nurture debate regarding violent criminals.