"Angel of Death" friend on learning his secret: "Most devastating moment of my life"

(CBS News) On "60 Minutes" Sunday night, Steve Kroft interviewed convicted killer Charles Cullen, known as the "Angel of Death." It was the first time in the program's 45-year history a serial killer was interviewed.

Cullen was a former critical care nurse, and on "CBS This Morning," a former nurse colleague and close friend, Amy Ridgway, discussed the moment she learned that Cullen was killing people.

"Angel of Death" killer nurse stopped, but not soon enough

"It was the most devastating moment of my entire life, at not really so much about knowing that one of my closest friends was murdering people, because how do you process that, but I hadn't protected these people," she said. "They were my patients. They were all of the patients at Somerset Medical Center of the nurses I worked with, and not only were we in charge of their medical safety -- we were in charge of them.

"They were completely vulnerable. We tied them down. We intubated them. We gave them medications. And they were completely at our disposal to do anything," she said. "And we were supposed to protect them, their bodies, their souls, and I couldn't -- I couldn't protect them."

Cullen was a friend with a "sardonic humor about him," Ridgway said, also describing him as self-deprecating and very shy. "I'm always very drawn to people like that, and he was great listener."

Ridgway, who helped police catch Cullen, said she dealt with a lot of guilt and was conflicted about helping police. "When I decided to help the prosecution, t was, 'I need to do this for the families of those victims and all of the nurses that I represented,'" she recalled.

Ridgway said Cullen has apologized to her since she began seeing him as she collaborates on a book about his case, but, she added, "He's never said that he's truly remorseful about what he did. I think he was sorry that he was caught."

Cullen confessed to killing between 30 and 40 patients, injecting them with drugs. And he may have killed many more. Cullen told Kroft his killing wasn't personal and that he thought he was helping his victims.

Cullen suggested several times that his actions were merciful, but the evidence doesn't support it. Elenor Stoecker, 60-year-old asthma patient, was recovering and in no pain when Cullen administered a fatal digoxin overdose. College student Michael Strenko, who suffered from an auto-immune disease, was recovering from what his parents called routine surgery to remove his spleen.

Kroft said in the "60 Minutes" report, "There were people that you caused to die, who were not near death, and not suffering that much."

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

Kroft pressed, "Can you give us anything? Can you give the families anything? Any explanation for how this happened and why this happened?"

Cullen said, "Like I said, I --I can't -- I just can't say that. It was more or less, you know, it felt like I needed to do something. And, I, I did. And that's not an answer to anything."

But Charlie Graeber, who spent seven years working on a book about the case called "The Good Nurse," said on "CTM" the killing was all about Cullen. "He's sometimes mistaken as a mercy killer. The 'Angel of Death' sort of suggests mercy. It was always about Charlie Cullen. It had nothing to do with anyone else."