But when the homicidal maniac enlists the help of his lover to lure in the vulnerable and hunt humans together, they become a strong and malevolent force.
Delivered into the hands of a pitiless psychopath, the victim can expect no mercy from his black-hearted accomplice who aids her lover's awful crimes to please him.
The most frightening serial killer couples have achieved worldwide infamy.
They include the Moors murderers Myra Hindley and Ian Brady, the House of Horrors killers Fred and Rosemary West and Perth's diabolical couple Catherine and John Birnie.
Their names bywords for evil, they had little chance of anything other than spending the rest of their lives in jail.
So how is it that one serial killer couple, possibly the most dangerous of all because of their charm and good looks, could soon be free to walk the streets?
In fact, the former wife, Karla Leanne Homolka, has been free from jail for years, living in relative obscurity under the pseudonym "Leanne Teale" invented for her by her depraved partner.
She is married and has three children.
Now Paul Kenneth Bernardo, aka Paul Jason Teale, has a chance of being released from prison on parole.
At a parole hearing in which Bernardo minimised his horrific crimes of raping and murdering virgin girls by blaming "stress and low self-esteem", he seemed to be reaching out to his now ex-wife Homolka.
After 25 years in prison, could the still handsome Bernardo be released to roam the streets and almost certainly find young females to rape, maim and kill?
As a couple, Bernardo and Homolka were made even more deadly by their good looks, charm and middle class status, which fooled police again and again.
Bernardo was a handsome, blond young professional whose charm masked the fact he was a savage, ruthless predator.
In the late 1980s he lived in the Toronto suburb of Scarborough in Canada where he worked as a junior accountant by day.
The assaults were rough and, as Mr Hazelton later described, the rapist seemed to revel in "degrading and demeaning (the victims with) terms such as b**ch, w**re, sl*t" as he raped them.
Over eight months, at least six women were attacked. The local media dubbed the man striking fear on their streets the "Scarborough rapist".
In October of that year, Bernardo met 17-year-old Karla Leanne Homolka at a convention and the pair ended up in a hotel room together that evening.
Homolka was from the small town of St Catharines, near Niagara Falls, 60km southeast of Toronto.
Although in possession of a strong will and a high IQ, she quickly fell under his spell.
Bernardo, 23, seemed the perfect man, magnetic, glamorous and from the big city.
Two days before Christmas, a 17-year-old girl was raped at knifepoint in Scarborough.
The rapes continued into 1988.
Bernardo raped a 15-year-old at a bus shelter, two girls aged 19 and one aged 22.
Acting on a tip from the wife of one of Bernardo's close friends, who told police Bernardo liked rough sex and anal sex, police interviewed him, but Bernardo was released.
Police set up surveillance and a hotline and eventually an 11th victim was able to give police a detailed description.
A sketch was released, and when friends started teasing Bernardo about how much it looked like him, he laughed it off.
In November 1988, police interviewed Bernardo again and he offered a DNA sample, although the science was in its infancy and very slow to produce results.
Bernardo was a master at hiding his violent secret, and then the attacks in Scarborough stopped.
Bernardo had moved from Toronto to live with Homolka's family, who liked him.
Soon afterwards, a 14-year-old schoolgirl was grabbed off the street in St Catharines in a brazen, broad-daylight attack.
As Mr Hazelwood put it, Bernardo by this time "believed he was all powerful … omnipotent".
He had also made Homolka aware of his desire to have sex with unspoilt, virgin girls, and that he had his eyes on her younger sister, Tammy.
Now an engaged couple Bernardo, 26, and Homolka, 20, made plans for him to take the virginity of her younger sister, Tammy, 15.
Homolka stole tranquillisers from the animal clinic where she worked.
On December 23, 1990, they stayed up drinking with Tammy after the Homolka parents, Dorothy and Karel, went to bed.
Karla fed Tammy eggnogs laced with tranquillisers and Bernardo and Karla videotaped themselves raping her.
Karla held a cloth soaked with the anaesthetic halothan over her sister's nose and mouth to ensure she wouldn't wake up, until the comatose teenager began to vomit.
Panicking, they called an ambulance that took Tammy to hospital where she died three hours later.
The official cause of death was accidentally choking on her own vomit after consuming alcohol.
It later emerged that Karla played Tammy's rape video and dressed up for Bernardo, posing in her dead sister's clothes.
As the Homolkas grieved, Bernardo and Karla moved out to a house 7km away in Port Dalhousie on Lake Ontario, where they began making plans for a big wedding.
On June 7, 1991, Karla invited home a 15-year-old girl she plied with alcohol and the tranquilliser Halcion.
Karla told Bernardo the girl was a "surprise wedding gift" and they both raped the unconscious girl all night, with the girl unaware the next day what had gone on.
She returned to the house two months later and stopped breathing during her drugged raping, but recovered and survived.
Meanwhile, on June 14, 1991, 14-year-old Leslie Erin Mahaffy had been out late before returning to her family home in Burlington.
The Year 9 student, daughter of an oceanographer and a teacher, was close to her family but had begun to rebel.
On the night in question she had returned home after drinking with schoolfriends until 2am and found the doors locked.
Leslie walked to a pay phone at a nearby milk bar to call a friend, and Paul Bernardo who was out on the streets followed her back home, wrapped his sweatshirt around her head and forced her into his vehicle.
The next day, her mother called police, believing Leslie Mahaffy was a runaway.
Instead she was being raped by Bernardo and Homolka who said they gave her a teddy bear to hold in between assaults.
Lying inland from Port Dalhousie, the man-made lake is a reservoir for Ontario's hydro-electric power system and a popular fishing spot.
As the waterline receded on Lake Gibson in midsummer, cement blocks surfaced at the edges.
One fisherman found blocks from which a set of limbs protruded.
Further up the lake, beneath a bridge, another fisherman spotted an enormous cement block, weighing more than 90kg, in which police would find a head and torso.
As forensic police recovered the body parts of the girl they had murdered, Homolka arrived in a horse-drawn carriage to marry Bernardo.
Guests at the lavish affair sipped champagne and dined on roast pheasant.
In wedding photographs, Bernardo looks the perfect groom, dressed in white tie and tails next to a petite Homolka in a white puffball dress and veil, holding roses.
The images would later lead to the pair being dubbed in some reports the "Ken and Barbie killers".
That afternoon they flew to Hawaii on their honeymoon.
While they were away, forensic police identified the dismembered body as missing teen Leslie Mahaffy and determined she had been strangled before being cut up.
The Kwik Mix brand cement used to encase was one of the police's few clues.
When Homolka and Bernardo returned from Hawaii, he blamed Homolka for not knowing about the receding water tables at Lake Gibson.
He began beating his new wife, but as the weeks rolled by and they weren't arrested, Bernardo asked Homolka to help him lure another young girl into their clutches.
After school finished for the day on April 16, 1992, Kristen French, 15, walked out from Holy Cross Secondary on her way home past the Grace Lutheran Church carpark.
Homolka and Bernardo were parked in their car, and when Karla got out with a map and asked for the girl's assistance, Kristen was totally fooled.
As she looked at the map, Bernardo grabbed her from behind and at knifepoint forced her into the vehicle.
Sitting behind in the back seat, Homolka grabbed Kristen's hair as they drove.
Kristen was known as reliable and took the same 15-minute route home each afternoon to care for her dog.
Niagara police searched the parking lot where they found evidence of an abduction, Kristen's shoe, and found witnesses.
For three days and into the forthcoming Easter weekend, Homolka and Bernardo filmed themselves torturing, raping and degrading Kristen and forcing alcohol down her throat.
The girl, who unlike Leslie Mahaffy was never blindfolded, was doomed.
Before going to Homolka's parents' house for Easter Sunday dinner, Bernardo took seven minutes to strangle French with a noose tied to Homolka's betrothal hope chest.
Homolka, who had beaten French with a rubber mallet when the girl tried to escape, watched on, then went to fix her hair.
On April 30, a man found Kristen French's nude body dumped 50km from the couple's house in a ditch in Burlington.
On December 27, he beat Homolka in the head and face with a flashlight, leaving her severely bruised.
She told workmates it was a car accident, but they called her parents who convinced her to press charges.
Homolka returned to the Port Dalhousie house to get the videos of all three murders she and Bernardo had committed, but couldn't find them.
Finally, in February 1993, Toronto police got the DNA results that matched Bernardo to the Scarborough rapist.
Bernardo was arrested and, through her lawyer, Homolka did a deal in exchange for a lighter sentence for her role in killing Tammy Homolka, Leslie Mahaffy and Kristen French.
She pleaded guilty to the manslaughter of her sister and the other girls' murders but received just 12 years – "five for Kristen, five for Leslie and two for Tammy".
The tapes of the vile crimes were still missing, but after six years on the loose it was finally up for sadistic psychopath Bernardo.
When he told his lawyer where the tapes were stashed – in a light fixture in the Port Dalhousie house – Homolka's true role was revealed in sickening detail.
Audio of the tapes of French and Mahaffy's terrible ordeals was played in court, causing a media firestorm.
Citizens picketed her court hearing with placards saying, "Rot in hell Karla".
Massive public criticism was heaped upon police and Homolka's "deal with the devil", with one observer calling her "just as much a psychopath as Paul".
In 1995, a jury convicted Bernardo of two counts each of first-degree murder, kidnapping, forcible confinement, aggravated sexual assault and one count of committing an indignity to a human body.
He confessed to raping and fatally drugging his 15-year-old sister-in-law and to another 32 rape-related crimes, including sexual assault and robbery, against 14 young women in Scarborough.
He was sentenced to life in prison but with a non-parole period of 25 years.
In July 2005, Homolka walked from prison after serving 12 years and ended up marrying her lawyer's brother and having three children.
According to the Facebook site Watching Karla Homolka, three years ago she was living in Chateauguay, Quebec, Canada, where she volunteered at the local school.
She went by the name of Leanne Teale.
Bernardo, aka Paul Jason Teale, is incarcerated in Ontario's Millhaven maximum security prison where in 2018 he was allegedly found with a jail-made knife.
A Canadian TV report of his court appearance on a charge - later dropped - of possess dangerous weapon to commit offence said Bernardo was "smiling and laughing" in the dock.
When Justice Patrick LeSage sentenced Bernardo in 1995, he told the sex killer: "You have no right ever to be released."
But under Canadian law, Bernardo is already eligible for day parole.
At a 2018 Parole Board of Canada hearing, Canada's National Post reported Bernardo "claimed to have discovered and confronted the psychological reasons for his sadistic sexual atrocities".
He blamed low self-esteem, stress, alcohol and "cognitive distortions".
Now aged in his mid-50s, Bernardo was making his first bid for parole.
The mothers of Leslie Mahaffy and Kristen French and one rape victim testified as to why he should not be let out.
Bernardo claimed in the hearing that rather than committing his terrible crimes for his thrills and sexual arousal, he had done so to make himself feel better, to ease his own internal pain.
Incredibly, Bernardo's self-serving case for his freedom included blaming victim Tammy Homolka's death for escalating his murderous ways.
The resulting guilt, he said, propelled him: "I failed the Homolkas. I failed Karla."
Dry-eyed, he told the parole board he cried "all the time" in prison in devastation at his past deeds.
The parole board knocked Bernardo back, but he's eligible again for parole this year.
He is believed to have committed other crimes, and in 2006 admitted to his lawyer he had committed at least 10 more sexual assaults in the 1980s.
One was of a 15-year-old girl for which another man had already served a sentence.
OTHER VICTIMS
Bernardo abducted and raped a young girl at the Homolka family home after Karla's parents had left town following Tammy's funeral in January 1991.
In the early morning of April 6, 1991, Bernardo abducted a 14-year-old girl at a rowing club who was momentarily distracted by a blonde women waving at her from a car.
Bernardo dragged her into bushes and sexually assaulted her.
Questions have also been raised as to whether Bernardo was responsible for the June 1990 murder of Elizabeth Bain in Scarborough, three weeks after his last known rape in the area.