Late one night in 1987, Paul Bernardo entered a Howard Johnson's restaurant where Karla was eating with a girlfriend. Her eyes widened. A handsome, six-foot-tall golden boy, Paul strode over to their table and introduced himself with an impish grin. Not only was he charming and attractive, he informed Karla that he was a trainee at a prestigious accounting firm. Alert to opportunity, she invited him to join them.

Paul, 23, liked the way that Karla flirted, her narrowed eyes cast sideways at him. More important, she seemed pliant and eager to please. As they talked, he watched for clues about how to seduce her. She yielded remarkably fast. Within two hours, they were in bed together.

They started seeing each other regularly. In Karla's mind, this was the man she'd been waiting for, the one who fulfilled her romantic . Although Paul had other girlfriends, Karla suspected nothing because he seemed to put her on a pedestal. She quickly learned that he had unusual sexual desires, and she didn't hesitate to indulge them. She offered him handcuffs and wrote him explicit notes, inviting him to select some "sick perverted acts" for her to perform.

Paul was initially pleased. Other girls had submitted to his sexual demands with reluctance, but Karla welcomed them. Nevertheless, his interest in her soon flagged, which alarmed her. She was deeply invested in the status that this attractive and seemingly successful older man conferred on her in the eyes of others. She had shown him off to her friends and family, who dubbed the couple "Ken and Barbie." Desperate to maintain his interest, Karla changed her hairstyle and clothing however he directed and continued to meet his increasingly perverse sexual demands. Their enmeshment was sealed when they got engaged two years after meeting.

As Karla planned a lavish wedding, Paul continued testing her commitment. He suggested that they bring a young girl into their lives. Karla refused at first, but once again Paul started to withdraw, so she panicked and consented to his demands. If sex with a young girl would keep Paul in her life, Karla decided she could accept it.

Even if, as Paul revealed, the young girl he desired was Karla's 15-year-old sister, Tammy.

Aroused by the fantasy of defiling a virgin, Paul had actually had his eye on Tammy for months, secretly masturbating while watching her sleep in the Homolkas' home. Paul declared that what he wanted for Christmas was Tammy's virginity, and Karla agreed to "give" it to him.

The night before Christmas, Karla slipped a tranquilizer into Tammy's rum-and-eggnog and then used halothane, stolen from the veterinary clinic where she worked, to keep the girl . In the basement of her home, the couple videotaped themselves as Paul raped and sodomized Tammy and Karla digitally penetrated her. Things took an unexpected turn when Tammy vomited, suffocated, and died. Unnerved, Karla and Paul put her clothes back on before calling emergency services.

After concocting a story that Tammy had gotten drunk at a family party and passed out, they held their breath while awaiting the autopsy results. The medical examiner failed to check Tammy's body closely enough to detect what had really happened. Free of any suspicion, they went ahead with their wedding as planned, despite the grieving Homolkas' wish that they postpone it. Not only did the criminal couple not mourn, Karla soon dressed in Tammy's clothes so that Paul could reenact the rape.

Paul's violent and sadistic impulses only mounted after Tammy's death. Six months later, he kidnapped 14-year-old Leslie Mahaffy, whom he and Karla raped, killed, and dismembered in the basement of their home. Ten months later, Karla actively helped him kidnap another girl, 15-year-old Kristen French, whom they also raped and murdered.

For Paul, these were just the latest and most gruesome in a growing tally of crimes he had already committed as a serial rapist, having carried out at least 14 other violent sexual assaults before and after meeting Karla. Having escalated from groping to raping girls—mostly in the area around Scarborough, Ontario—Paul had become known to the public and the police as the "Scarborough Rapist."

After parts of Leslie Mahaffy's body were found encased in concrete and dumped in a nearby lake, and Kristen French's body was discovered in a ditch, authorities surmised that the girls' killer might be the Scarborough Rapist. Investigators with expertise in decided to make a public televised appeal, knowing that such predators often had wives or girlfriends who could recognize the traits described and alert the authorities.

What they didn't predict was that the partner of this predator was actually a participant in the crimes.

Paul and Karla watched the televised appeal together. Paul dismissed the report, believing that the description was inaccurate and that he would remain free. Karla disagreed. She could sense that the authorities were beginning to close in on them.

A Match Made in Hell

It's often mystifying how a seemingly ordinary person like Karla can become a participant in monstrous acts. Sometimes an accomplice is just at the wrong place at the wrong time. But many times the follower willingly accepts a role as half of a murdering duo.

More than a fifth of serial killers operate in teams, most involving two offenders. One is invariably a dominant figure who looks for someone deeply insecure, often exploiting youth, neediness, mental instability, or low , according to Eric W. Hickey, a and the author of Serial Murderers and Their Victims. In his analysis of more than 500 serial killer teams, each had one person who maintained psychological control.

The murderous duo relationship is characterized by a strong interdependence in which both parties need something critical from the other, explains Al Carlisle, Ph.D., a former prison psychologist and the author of I'm Not Guilty: The Case of Ted Bundy. "The dominant person needs the follower's total loyalty in order to validate him- or herself," says Carlisle. "The subservient follower needs the power and authority of the dominant person, so he or she attempts to become that person's shadow and to mirror the dominant person's beliefs and ethics. Each receives justification from the other."

Gregg McCrary, a behavioral and former FBI agent who worked on the investigation of Bernardo and Homolka, says that predators like Bernardo often have a highly acute ability to detect potential co-conspirators. "There's radar, gaydar, and maybe, mur-dar," McCrary says. "It resembles the phenomenon wherein normal people meet and decide that they're going to get along, but with these couples, it takes a dark turn. They vector in on each other, sensing the excitement of a kindred spirit. It becomes electric."

Once a person's "mur-dar" homes in on a likely partner, the next step is typically a test. "The process is exploratory," McCrary explains. "Predators are like sharks waiting to see who will swim by and take the bait. If the other person reacts badly, it's easy to shrug the incident off as a joke. When they find one who responds the way they're hoping, they think, This is someone who can do this with me. They might say, 'What would you think if I were a rapist?' If the person thinks it's cool, they can take the next step."

What the dominant predator ultimately wants is complete psychological and, in the case of a sexual sadist, physical control. Achieving such dominance can preemptively shift an accomplice's potential moral conflict in the predator's favor. The partnership acquires greater emotional significance than any personal moral code that might have prompted the accomplice to resist committing a crime.

Such is the deadly mind game that played out with Karla and Paul. His ability to rapidly recognize the traits he sought in Karla—a young woman who showed keen interest in him and willingly participated in his sexual deviance—meant he had locked in on a potential accomplice. Once they were , her desperation for his attention and the fulfillment of her own romantic fantasies allowed him to manipulate her into ever more odious acts.

No other girl Paul had previously seduced had been as willingly deviant as Karla. Some had sensed danger and quickly ended the relationship. But Karla took whatever Paul dished out. Little by little, his —his twisted sexual obsessions—and his ability to manipulate and control Karla dissolved her moral frame. By submitting to degrading sex acts and obeying his every order, she ceded authority to him over every aspect of her life. Her dependence and desperation to hold on to Paul made her ever more malleable. Even after orchestrating her own sister's rape, which led to her death, Karla remained loyal.

No one would ever have suspected such behavior from Karla. No one, perhaps, except Paul. From the time they met, his mur-dar had sensed Karla's inner green light. From there, it was but a series of steps to turn an average girl into a dangerous criminal.

Devils in the Details

Former FBI Special Agent Robert Hazelwood and Janet Warren, a professor of clinical medicine, published a study in 2002 that offers insight into the development of killing teams. They interviewed 20 wives or girlfriends ("compliant accomplices") of sadistic sexual predators, four of whom had observed or participated in murder. Despite the small sample, their study, reported in the Journal of Family Violence, remains the standard for understanding an accomplice's experience of assimilation.

Most of their subjects were middle-class women with family backgrounds of physical or whose male partners had transformed them through a series of steps. First, the men identified a vulnerable person. Then they seduced her with gifts and praise while isolating her and eroding her self-preserving boundaries and sexual norms. Some also used a process common to abusers: They layered cruelty and with kindness and affection. They met the women's needs before inflicting pain.

The predators' initial show of "love" was a ploy to acquire information for leverage. Then they introduced a sexual act that was outside the woman's experience. Obtaining compliance, they added others until deviance became routine. They increased their partner's dependence and restricted outside contact. Already fragile, these women came to need the males for self-worth, companionship, and life direction.

"Most had dependent and low ," Hazelwood says. "Those I talked to indicated abhorrence but also complete intimidation and subservience to the male."

Accomplices' motivations to participate in murder were complex but tended to range from love to for their life or that of their children. Some adopted distancing mechanisms, such as or just looking away when violence occurred. Generally, they had to agree to something they fundamentally loathed, like harming or killing another person, in order to acquire something they needed or craved—the security, social status, and love they had become dependent on from their male partners.

Paul Bernardo followed this precise pattern with young women he dated, including Karla Homolka. Initially, he would be attentive, generous, and caring. He presented himself well to the girls' families. Only when he thought he possessed a girl would he engage in verbal and physical abuse. Other girls had become frightened and ended the relationship, but Karla reacted by becoming fully compliant. As Bernardo gradually devalued her and then physically hurt her, she lost and grew even more dependent. She kept hoping that her "prince" would change back into the loving man he once had been.

It's very rare for a female in male-female killing teams to take the lead, but it does happen. In 2013, in Pennsylvania, Miranda Barbour, 19, proposed to her new husband, Elytte, that they kill someone together, assuring him that she had gotten away with murder before. Married for just three weeks, they used a Craigslist ad to meet and lure a 42-year-old man named Troy LaFerrara with the prospect of a sexual encounter. At Miranda's instruction, Elytte hid under a blanket in the backseat of their SUV as Miranda drove to a mall to pick up their victim. Once Troy was seated, Elytte forced a cord around his neck to incapacitate him while Miranda stabbed him. Then they dumped his body, cleaned up the SUV, and went to a strip club to celebrate Elytte's 22nd birthday. When they were caught, Miranda put forth a story about self-defense. Elytte, the weaker partner, admitted their to the authorities.

Nabbing the Fragile Mind

It's not always a through which a dominant aggressor manipulates a weaker person toward murder. In the case of John Allen Muhammad and Lee Boyd Malvo, who for three weeks in 2002 randomly shot 13 people in the Washington, D.C., area, killing 10, the relationship was akin to that of a father and son, with the younger man desperately in need of paternal love.

Malvo, 17, had grown up in an abusive and neglectful family. He knew of no other way until he saw how John Allen Muhammad, 42, acted around his own son. Muhammad's actions toward John Jr. had a profound effect on Malvo. When, without being prompted, Muhammad bought drinks for other kids who were playing with his son, it offered a glimpse of what Malvo desperately sought: attention and love. Muhammad cultivated an emotional connection with Malvo, offering him praise and support; eventually there was nothing Malvo would not do for this father figure.

Anthony Meoli, a psychologist and the author of The Diary of the D.C. Sniper, explains that Malvo "viewed their love as akin to a master and disciple. If he chose to not obey, John's love would fade away, just like that of his father and mother, who had abandoned him many times before. In order to ensure that Muhammad would always be there as his father figure, he obeyed his every word, even to his own detriment."

Malvo himself admitted as much after the fact, when asked about how Muhammad had been able to command such powerful loyalty. "He didn't really have to instill it," Malvo told Meoli in an interview. "There was a hole there for the last 15 years from my real parents that I was trying to fill. [Muhammad] just had to be there, show up, and be consistent. He was bad, he was terrible, but he was there."

Same-age friendships can also become cauldrons for a killing team. In 2009, Christopher Gribble, 20, accompanied Steven Spader, 17, and two other friends into the home of Kimberly Cates in New Hampshire. Spader hacked her to death with a machete while Gribble stabbed Cates's 11-year-old daughter, Jamie. The other two young men, sickened, stood by. Jamie played dead, which allowed her to survive and get help. All four boys were arrested the following day.

Gribble admitted to police that he thought the experience was "cool." His only regret, he said, was that the girl survived. He seemed upbeat as he led investigators to where he and Spader had buried the weapons.

Kent Kiehl, a neuropsychologist and the author of The Whisperer: The Science of Those Without , evaluated Gribble to help his attorney learn why this quiet, unassuming kid had committed such a vile act. Kiehl found the home-schooled Gribble to have led a sheltered life in a devoutly Mormon household. He was socially awkward with a low IQ. "It was like talking with a 10-year-old," Kiehl says. "He had limited social experiences, as if he'd lived inside a bubble."

It seemed likely that Spader, who had a history of disturbed criminality, had strongly influenced the suggestible, immature Gribble. "My clinical sense," Kiehl says, "is that this kid just had a fragile mind. You can plant an idea in a fragile mind and get such a person to do anything."

What About Karla?

Unlike Lee Boyd Malvo and Christopher Gribble, Karla Homolka was not fragile. She came from a stable home with loving parents. She had frequent arguments with her father, but she had not been physically or emotionally abused. By all accounts, she was intelligent. "She certainly did not suffer from low self-esteem," McCrary observes. So how could Paul Bernardo have known she could be so easily lured?

The age difference helped; she believed Paul was superior to boys her own age. But Paul's mur-dar had detected something more fundamental. In fact, Karla had always had cracks in her good-girl image. Before she even met Paul, she liked to "talk dirty." She had bullied people she thought looked weird. She once tossed a friend's pet hamster out a second-story window in a homemade parachute and acted dismissive when the animal died from the fall.

She craved excitement, broke rules, and showed little compassion for others. Without Paul, Karla might have continued on simply as a callous person, never killing anyone. But her dependence on him, her self-centered , and her immaturity made her an easy person to manipulate according to his needs.

In the end, though, Karla was quick to save herself. The same traits that draw a dominant predator to a submissive partner can also jeopardize their enterprise.

"It speaks to their lack of self-esteem," says Eric Hickey of the accomplices' mindset. "They're so desperate to belong, and when things fall apart, they realize they were used. They didn't really have a relationship, so they start to distance themselves. If they thought that the other person cared about them, they would be less inclined to protect themselves, but when they see the reality, they feel less allegiance."

And this is what happened, finally, to Karla. In January 1993, Paul beat her so badly it sent her to the emergency room. The police charged Paul with assault, and Karla sought refuge with her family. In February, three samples from rape kits for the Scarborough victims were matched to Paul. When the police came to question Karla about the rapes, her loyalty to Paul collapsed.

Karla retained an attorney and was soon sitting across from task force detectives and telling her tale. Peering at them from under her long bangs, she presented herself as a hapless young woman who had been too scared to stop or leave her brutal, wife-beating husband. Leaving out any mention of Tammy, she described the assaults and murders of Mahaffy and French. She had been a reluctant participant, she said, who tried to offer the girls some comfort in their dying moments.

In a soft voice, Karla persuaded her interrogators that she had been trapped, that love had transported her into Paul's violent world. "He treated me like a princess," she told them. "You have to understand, I liked him back then. He never bored me like the others." Now, she said, she wanted him to rot in prison.

Bernardo was arrested and charged with 42 criminal counts. For her and a plea of guilty to manslaughter, Karla was sentenced to two 12-year terms to be served concurrently. Only when the couple's videotapes later surfaced did investigators realize Karla's . Rather than comforting the victims, it was clear that she had participated in her husband's assaults, and an exhumation revealed the true cause of Tammy's death. But the deal was done.

Bernardo is currently serving a life sentence in prison. Although he was eventually stopped, three girls are dead because his dark instincts detected an accomplice for deviance and murder.

Two's Deadly Company

Notable killing duos and the dynamic that sealed their partnerships.

Nathan Leopold & Richard Loeb: As privileged young men, Leopold and Loeb shared a disdain for moral codes. Loeb, a crime buff, could easily manipulate Leopold's infatuation with him. In 1924, they decided to commit the "perfect" murder. To prove their superiority, they killed 14-year-old Bobby Franks. When suspected, though, each turned on the other, and Leopold admitted he would have done anything to please Loeb.

Bonnie Parker & Clyde Barrow: When Parker met Barrow in 1930, they were instantly smitten. Her admiration for his reckless revealed her tolerance for violence. Her enthusiasm and loyalty empowered his spree of bank robberies and murders.

Charles Starkweather & Caril Ann Fugate: Fugate was just 14 in 1958 when Starkweather, 19, took her on a killing spree across Nebraska that claimed 10 lives, starting with her own family members. He liked that she never questioned him and let him be whoever he wanted. Her youth and immaturity made her the compliant companion he desired.

Ian Brady & Myra Hindley: Brady believed that superior men could transcend cultural mores and do what they pleased, including murder.

In the early 1960s, Brady exploited 18-year-old Myra Hindley's yearning for an exciting life and her infatuation with him to help him kill five people, including three children, in the area around Manchester, England.

Dean Corll & Elmer Wayne Henley: In Houston, in the early 1970s, Corll, an established businessman with a sadistic taste for rape and torture, bribed two adolescent boys from broken homes to help him lure as many as 30 other boys for rape and murder. Fifteen-year-old Henley, initially introduced to Corll as a potential victim, proved easy to manipulate with and drugs. Henley brought his own buddies for Corll's abuse and even killed some himself.

Douglas Clark & Carol Bundy: In 1980, Clark spotted Bundy in a bar in California. He viewed the overweight, bespectacled woman who looked as an easy mark.

He moved in with her and made her so desperate to keep him that he easily persuaded her to participate in crimes. As the "Sunset Strip Killers," they murdered several women together before Bundy killed a man on her own.