The 'Angel of Death', serial killer Charles Cullen, has appeared on 60 minutes in the US for his first ever public interview. Courtesy CBS

A CRITICAL care nurse turned serial killer who admits he murdered up to 40 patients has spoken publicly about his crimes for the first time in a chilling interview.

Charles Cullen killed his victims by injecting them with lethal drugs or poisoning their IV bags. He murdered patients for 16 years, shifting hospitals when his employers grew too suspicious.

"Forty is an estimate (of how many people I killed). I gave a number between 30 and 40. I think I have identified, you know, most of them," Cullen told CBS' 60 Minutes this week.

That may be a conservative estimate. Crime author Charles Graeber, who investigated the case for seven years, believes the real death toll from Cullen's crimes could be in the hundreds. The US media dubbed him "The Angel of Death".

Cullen was arrested in 2003 and he is currently serving a sentence of life without parole in New Jersey state prison.

Why would a nurse poison his patients?

"I thought that people weren't suffering anymore. So, in a sense, I thought I was helping," Cullen said.

"I mean, my goal here isn't to justify. You know what I did there is no justification. Um, I just think the only thing I can say is that I felt overwhelmed at the time."

"I felt like I needed to do something. And I, I did."

Cullen tried to kill himself "at least 20 times" as a young man, as he dealt with alcoholism, bankruptcy and divorce. He received psychiatric treatment, but continued to work as a critical care nurse.

"I tried to kill myself throughout my life because I never really liked being who I was. Cause I didn't think I was worthy of anything," Cullen said.

The warning signs were there, and Cullen himself says his killing spree should have been stopped several times.

Cullen was forced from his first job, at St Barnabas Medical Center in New Jersey, when he became management's main suspect in a series of poisonings. The overdoses stopped after he left, but no subsequent employers were alerted to his history.

"I think you can say I was caught at St Barnabas, and I was caught at St Luke's (hospitals). There's no reason that I should've been a practicing nurse after that," Cullen said.

Instead of reporting Cullen to the police, the staff at St Luke's Hospital told him that if he left, they would give him neutral references.

The 60 Minutes interview ended with the reporter asking Cullen whether he was sorry.

"Yes. But like I said, I don't know if I would've stopped."