American serial killer Ted Bundy brutally murdered dozens of young women in the 1970s.

Many of them were kidnapped, raped and assaulted before their death. One was just 12 years old.

So when Zac Efron, the teen heartthrob from Disney's High School Musical, was cast to play the killer in Joe Berlinger's new film Extremely Wicked, Shockingly Evil, and Vile, you can understand why some were apprehensive.

Efron said he was excited to be playing the role. ( Instagram: Zac Efron )

People worried Efron would be too charming in the role, and fans would find themselves lusting after a notoriously violent criminal.

"People gonna be falling in love with a serial killer," one person wrote on Twitter.

Then Netflix dropped its new docuseries, Conversations with a Killer: The Ted Bundy Tapes, and ran into a similar problem.

The series paints a disturbing picture of Bundy, through interviews with the detectives that discovered his crimes, archival footage of his trials, and audio recordings he made while on death row, though much of the chatter online was around his good looks.

It prompted this response from the streaming giant:

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But this is nothing new…

Bundy was an attractive, college-educated and charismatic man.

He was a former law student, had worked in politics, and was described by those who knew him as very likeable with a good sense of humour.

So when he was caught by police and went on trial for murder, many couldn't picture him carrying out such brutal crimes.

His handsome looks made him somewhat of a celebrity and he attracted many female fans.

Ted Bundy often joked with reporters and in the courtroom. ( Supplied: Netflix )

Margaret Good, one of his early defence lawyers, recalls in Berlinger's docuseries how women would turn up to court and pass her handwritten notes to give him.

"He just doesn't look like the type to kill somebody," one young woman told a reporter outside the courtroom.

Carole Ann Boone, one of Bundy's close friends, even agreed to marry him just moments before he received a death sentence for murdering middle school student Kimberly Diane Leach.

She believed he was innocent and had a child with him while he was on death row.

But eventually Bundy confessed to his crimes just before being executed by electric chair in Florida, 1989.

So what is it about violent criminals that people find attractive?

Charles Manson, the cult leader who directed his devotees to murder seven people, had a fan club.

People are still fascinated by the Milwaukee Cannibal, Jeffrey Dahmer, who killed and ate 17 people.

And Penn Badgley, the former star of Gossip Girl, had to fight off fans who loved him as a murderous stalker in Netflix's TV series, YOU.

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So what is it about violent offenders?

Katherine Ramsland, a professor of forensic psychology at DeSales University in Pennsylvania and the author of The Human Predator, said in Bundy's case it was his good looks and charm.

"Many women thought Ted Bundy was handsome," she said.

"That he was in law school and knew how to dress well was also a plus."

Bundy is remembered for his piercing eyes. ( Supplied: Netflix )

But Dr Ramsland said offenders — both men and women — didn't have to be physically attractive to attract admirers.

"They can be bold and confident," she said.

"[Bundy] had the confidence, charisma, daring, and resourcefulness of an alpha-male personality that projects a sense of protection.

"In addition, he had a nervous energy and predatory look that attracts some types who like the edginess of a person who can plan and carry out violent acts.

"In their minds, they thought, he might kill other women but he'd be good to me."

Sometimes just the media attention can be enough.

"Lots of people are drawn to 'celebrities', no matter what they've done," Dr Ramsland said.

And others like the "edgy sense of violence in getting close to convicted killers", she said.

Hybristophilia, for example, refers to when people are sexually aroused by partners who are known to have committed an offensive or violent act.