A former nurse turned FBI profiler interviewed dozens of killers. She explains what the most dangerous ones have in common.

Candice DeLong speaks onstage during A True Crime Experience Live at Gotham Hall in 2023. Ilya S. Savenok/Getty Images for Wondery Candice DeLong shifted from psychiatric nursing to a 20-year career in the FBI profiling killers. DeLong's career insights show that lack of guilt defines the most dangerous serial killers. DeLong found that success after a first murder often encourages serial killers to continue killing. Candice DeLong didn't plan to spend her career interviewing killers. She started as a psychiatric nurse, helping patients at Northwestern University Hospital in Chicago, before an FBI agent attending one of her lectures suggested she consider joining the bureau. DeLong entered the FBI at age 30, and spent the next 20 years investigating violent crime and profiling offenders. She later interviewed convicted murderers for television, giving her a front-row seat to dozens of killers over the course of her career. That unusual career path shaped the way DeLong thinks about violent crime . Her years as a psychiatric nurse taught her to understand severe mental illness before she ever carried a badge. Later, she helped investigate major cases, including the FBI's operation to arrest Ted Kaczynski, the Unabomber. Theodore 'Ted' Kaczynski (in orange) is guided to his arraignment by federal marshals in 1996. Michael Macor/The San Francisco Chronicle via Getty Images Through decades of interviews, she says one trait consistently separated the most dangerous killers from everyone else: They kept killing because they didn't experience the guilt that stops most people. The killers who never felt guilt "I've never interviewed or studied a serial killer that was not a psychopath ," she said. She stressed that psychopathy isn't the same as psychosis. Someone experiencing psychosis may lose touch with reality through hallucinations or delusions. A psychopath, by contrast, understands what they're doing but lacks remorse and empathy , she said. Candice DeLong speaks onstage during A True Crime Experience Live at Gotham Hall in 2023. Ilya S. Savenok/Getty Images for Wondery That emotional detachment allows serial killers to repeat their crimes. "They do anything they can to relive the moment," DeLong said. Some revisit crime scenes, while others keep souvenirs from victims to recreate the experience. Some convicted killers spent years trying to convince themselves they were innocent, she said. "Sitting in a prison realizing you're there for the rest of your life because you took someone else's life has to be pretty hard," she said. "It seemed that a lot of the people I was talking to were like, if they told the lie enough times, they'd believe it." What made the worst killers different DeLong said many murderers had traumatic childhoods that included neglect, physical abuse, sexual abuse, or severe mental illness. But those experiences alone didn't explain who became a serial killer. Instead, she says the biggest distinction was what happened after the first murder. Sometimes the first killing wasn't planned, she said. If the killer avoided capture or gained financially from the crime, that success could reinforce the behavior. "They think, 'Well, that was easy,'" DeLong said. "Getting away with murder is reinforcement for doing it again." She believes serial killers become more dangerous as they gain confidence. Each successful crime lowers their anxiety and strengthens the belief that they'll escape again. Over decades of interviewing offenders, DeLong said one lesson has stayed with her. "The average killer in prison is not someone drooling at the mouth," she said. "They're not monsters as such." Still, she believes some offenders should never return to society. "As difficult as it must be to accept that the person you're looking at in the mirror is a killer," she said, "there are certain crimes and certain people that I think, even in their mid-80s in prison, should never get out. There's no reason to believe if they have the opportunity to kill again that they won't." Read the original article on Business Insider