Charles Cullen, a serial killer who went undetected for 16-years, spoke with 60 Minutes about his crimes. The former nurse-turned-serial killer is accused of giving patients overdoses of medication, killing them in the process. It is believed the serial killer murdered at least 40 people in New Jersey and Pennsylvania.

Charles Cullen was caught in 2003 and sentenced to 6 life sentances in 2006. In his interview Cullen said he probably would have kept killing were he not caught.

60 Minutes aired a show last night in which correspondent Steve Kroft and producer Graham Messick interviewed the serial killer accused of murdering patients under his care as a nurse.

"It takes a really subversive, devious mind to break all the rules, and actually use that to commit the worst crime of all," Messick said in a 60 Minutes Overtime video.

"When we interviewed him -- and he's been in this particular prison for nine years -- I thought we were gonna see somebody who has really come to grips with what they'd done. He hasn't," Messick added.

Journalist Steve Kroft spoke with Charles Cullen, unsure if the serial killer would sit there and answer all of his questions.

"He ended up talking for 80-minutes," Kroft said.

Kroft asked Cullen, "Did you consider yourself to be a good nurse?"

"For the most part yeah," a sullen and worn out looking Cullen responded. During his 60 Minutes interview Cullen displayed very little emotion. The only facial expression he made was one of annoyance.

On CBS.com, 60 Minutes posted a behind the story video in which the producers spoke about putting the story together. One of the associate producers said that their goal was to make sure the families of the victims were represented properly.

"We tried really hard in a number of ways to give the families something about why he did it," said Kroft. "

Kroft asked Cullen why he did what he did but did not get a clear answer. Kroft did his best to try and "engage Cullen in conversation" in an attempt to find out why he killed 40 people.

In the 60 Minutes interview, Cullen said the first time he killed a patient was when he was a nurse in the burn unit. Cullen said he killed the person to keep them from suffering.

"The patient was John Yengo, a judge from New Jersey, who was suffering from a severe case of sunburn, until Cullen injected him with a fatal overdose of lidocaine," says CBS News.

CBSnews.com has transcripts of the interview between Kroft and Cullen:

Steve Kroft: Was it personal?

Charles Cullen: No, no.

Steve Kroft: Did you get pleasure out of it? Satisfaction? Relief?

Charles Cullen: No, I thought that people weren't suffering anymore. So, in a sense, I thought I was helping.