'The Good Nurse': How Amy Loughren Helped Put Her Serial Killer Colleague Behind Bars: 'Nurses Are Badass'

In 2003, while working in the Intensive Care Unit at Somerset Hospital in New Jersey, Amy Loughren always looked forward to her shifts alongside fellow nurse Charles Cullen.

"He was funny," Loughren tells PEOPLE. "We bonded right away and became friends."

While Cullen seemed to be a devoted caregiver, detectives investigating mysterious deaths at the hospital would, with Loughren's help, expose him as one of the most prolific serial killers in American history. After Loughren discovered the truth about her friend, she agreed to secretly record conversations with Cullen for Somerset County Police Department investigators and eventually persuaded her former pal to confess to killing patients by injecting their IV bags with lethal doses of insulin and heart medications.

The true story of how Loughren put an end to Cullen's killing spree is told in the movie The Good Nurse, starring Jessica Chastain as Loughren and Eddie Redmayne as Cullen. The film, inspired by the 2014 Charles Graeber book The Good Nurse: A True Story of Medicine, Madness, and Murder also stars Nnamdi Asomugha as Detective Danny Baldwin and Noah Emmerich as Det. Tim Braun.

"It's just not in my nature to betray one of my friends, but of course I knew I had to," says Loughren, 57, of Cullen, who is serving life in prison for his crimes. "What I love about nursing is that I could protect the vulnerable — and I'm a badass nurse."

In 2003, Baldwin and Braun visited Somerset Medical Center to investigate abnormal lab reports after the deaths of several patients. They interviewed staff and right away Baldwin noticed something special about Loughren.

"She came across as a strong-minded, intelligent individual, so I rolled the dice and revealed some of our findings to her," recalls Baldwin, who is now working as a private investigator.

Baldwin showed Loughren a record of the names and dosing information of drugs that her colleague Cullen had withdrawn while on duty in the ICU. "It was very, very obvious there was something not right," says Loughren.

A lightbulb went off and Loughren began putting the pieces together. She knew she had to help stop Cullen before he could hurt more patients. But before she could begin, the single mother of two daughters wanted to check with her oldest daughter, Alex, who was 11 at the time. "I told her, 'Our lives might completely change. I don't know if I can do this to you.' And Alex said, 'Mom, he's murdering people.'"

With her daughter's permission, she began working closely with a team of dedicated detectives and assistant prosecutors on the case.

She agreed to wear a wire and meet Cullen at a restaurant, where she hoped to coax him into admitting his crimes while detectives waited outside. "I was terrified," she says. She was also morally conflicted. "I was wrestling with how much I still cared for him. He was my friend. I didn't know the murderer."

That soon changed. After some casual pleasantries, Loughren confronted Cullen about the sudden deaths he had caused at the hospital and offered to go with him to the police station to turn himself in. Cullen's response haunts her to this day. "He sat straight up. The color of his eyes changed. He put a smirk on his face and said, 'I'm going to go down fighting.' "

He was arrested and brought to the police station, where Loughren convinced him to start talking. After his confession, he was convicted of 29 murders and was sentenced to 11 consecutive life sentences in 2006. But some experts estimate that Cullen could be responsible for as many as 400 murders, based on suspicious circumstances in the deaths of patients at hospitals where he worked in New Jersey and Pennsylvania, something they can't prove without further admission from Cullen.

Chastain tells PEOPLE that Loughren is an inspiration. "So often in films about serial killers, we're celebrating aggression as the way to stop violence, and rarely do we acknowledge love and understanding can heal that pain. Amy didn't use any aggressive tactics. She used her heart to find a way forward," says Chastain. "I found myself really moved by Amy's way of speaking to people — to her patients, her daughters, Charlie. There's a scene at the end of the film in particular where she approaches Charlie with humanity, compassion, empathy, and love — that really is her superpower. It's what allows for this resolution to come about and for people to start to heal."

Asomugha points out that, "Danny and Tim were also superheroes. A lot of doors closed on them but with the help of Amy, they were able to open them. It's a heroic thing that they did."

Loughren says the new film helped her heal from the past traumatic events. "Watching the movie gave me permission to be proud of myself," she says. "I showed up as a mom. I showed up as a nurse. I showed up as a friend. The only reason Charlie is not still murdering is because of my friendship with him."

The Good Nurse is currently in select theaters and airs worldwide on Netflix Oct. 26.