Ted Bundy wasn't always Ted Bundy.

The serial killer who admitted to the murders of 30 people in the 1970s was born in Burlington in 1946 as Theodore Robert Cowell to Eleanor Louise Cowell. According to Ann Rule's book, "The Stranger Beside Me," his mother claimed to have been seduced by a "sailor" despite records showing the biological father to be named an Air Force veteran named Lloyd Marshall.

Bundy's story is being told again in the upcoming Netflix movie "Extremely Wicked, Shockingly Evil and Vile," starring Zac Efron. The film centers on Bundy's girlfriend Liz Kendall (Lily Collins) as a living victim, with a small role for Bundy's mother Louisa (Forba Shepherd). It does not focus on Bundy's childhood.

A toddler experiencing violence in the home

After his birth in Vermont, Cowell initially left her young son at the Elizabeth Lund Home for Unwed Mothers, now known at the Lund Family Center. She later brought him to live with her and her parents in Philadelphia.

Rumors have long persisted that Bundy's grandfather, Sam Cowell, was actually his biological father. In the home, he grew up hearing his mom referred to as his sister and told to call his grandparents "Mother" and "Father."

Rule wrote that Bundy both adored and respected his grandfather and "clung to him times of trouble." In Netflix's 2019 docu-series “Conversations With A Killer," journalist Stephen Michaud said the man may have abused the young boy, as well.

“This, of course, this illegitimacy issue is, for the amateur psychologist, it’s the thing,” Bundy said in a clip featured in series. “I mean, it’s so stupid. It just bugs the s--t out of me. I don’t know what to do about it.”

30 years since his execution: Listen to 'Hunted,' a podcast about Ted Bundy's time terrorizing Colorado

But according to a report by a professional psychiatrist who examined Bundy a day before his execution, he suffered from manic-depressive illness which gave him violent mood swings. Dr. Dorothy Otnow Lewis wrote that the illness could be traced to his time living in Philadelphia, adding that his grandfather was "an extremely violent and frightening individual" who kicked dogs, swung cats by their tails and beat people who angered him.

Bundy told Lewis before his death that his access to his grandfather's pornographic magazines at a young age contributed to his violent behavior. In a 1987 competency hearing, Lewis reported that Bundy's aunt, then 15, woke up from a nap to find kitchen knives around her. She woke up to find her nephew Ted at age 3, by her bed smiling at her.

According to the FBI, childhood abuse and neglect have the potential to create an increased risk of future violence. Genetics and environment can also work in tandem to result in such behavior.

“Genetics loads the gun, their personality and psychology aim it, and their experiences pull the trigger,” said FBI profiler Jim Clemente.

But not all who experience childhood trauma act out in the same way or to the extreme that Bundy did.

The Bundy name and bitterness towards parents

Bundy's mother left Philadelphia, moving across the country to Washington State with her son when he was four-years-old. There, she changed his last name to Nelson before she met and married Johnny Bundy. The man formally adopted his wife's son, and Ted Nelson became Ted Bundy.

Family friend Sandi Holt recounted in the Netflix series that Bundy's mother and adopted father were religious and very involved in their children's lives. She said that as a boy, Bundy was teased for having a speech impediment and couldn't quite the get the hang of activities other boys were doing.

"He had a temper. He liked to scare people," Holt said.

As he grew older, Ted Bundy reportedly grew bitter toward his family for them not having as much money or possessions as others in his neighborhood. He also was said to have found his birth certificate and discover that Johnny was not his birth father.

Adoption records sealed for 99 years:What Ted Bundy's birth certificate shows about Vermont adoptions in the 20th century.

Although there are conflicting reports about when he may have found the document, friends remember his resentment towards his parents growing.

"When I made light of the situation, he said 'Well, it's not you that's a bastard.' He was bitter when he said it." said Terry Stowick in an interview in the book, "The Only Living Witness."

'He never really knew who he was'

Stowick added that in high school, Bundy became more socially withdrawn and was "very sensitive" with built-in insecurities. In college, he met a woman he seriously dated, but she broke it off. Bundy responded by traveling the country, including a stop in Philadelphia and doing a semester at Temple University to be close to his grandfather.

Bundy told Rule that he missed his grandfather, Sam Cowell, terribly when the family first moved west and thought of him fondly, describing him as a 'Santa Claus' type character despite his propensity for violence in the home.

"(Bundy) was so intense and disturbed when he said he never really knew who he was, or whom he belonged to," Rule wrote.

'Not really glorifying him' Why a Ted Bundy survivor doesn't 'have a problem' with Zac Efron's charming portrayal.

In research published in Joel Norris' book Serial Killers," the cycles of violence are described as being generational. A separate 2017 study found that serial killers who experienced childhood abuse tended to sexually assault victims before killing them.

In 1976, Psychologist Al Carlisle conducted a psychological assessment of Bundy. During a 2018 interview with TV network A&E, Carlisle said Bundy developed his psychopathic personality over time, shutting down feelings of guilt for his actions. He says the serial killer became the author of his own creation.

"He got addicted to the idea of this possession of the essence of the victim," Carlisle said. "Bundy created himself through poor choices."

Nate Chute is a producer with the USA Today Network. Follow him on Twitter at @nchute.