David Yarno was born in Liberal, Kansas and was raised in an abusive household, his father Donald being a violent alcoholic who would regularly beat him and his mother, Maggie. After Maggie divorced Donald and got a restraining order against him, she and David moved to Lawrence, Kansas, where David befriended Becker Chapin at his new school. Becker had a much more normal upbringing, with his parents, Nick, a mechanic, and Danielle, a secretary, having a steady relationship. The much larger Becker protected David from bullies and, as their friendship grew, they became more detached from their parents and began committing petty crimes together, eventually being arrested for arson. After high school, the two moved out of their parents' homes and into an RV, which was kept parked on a secluded patch of land owned by David's uncle. In October of 2004, the two, having gone to film school, passed in their term project, an exploitative horror film, early, expecting it to be judged a work of art. Written by David, with Becker directing, the film was deemed a disaster by their professor, being poorly directed, terribly acted and consisting almost entirely of scenes ripped straight from other horror films. The failing grade caused Becker to throw a fit in class, threatening to kill the professor before storming off and dropping out with David. Obsessed with proving themselves, David, when Becker suggested they create another film, decided to film real murders for use in it, recreating death scenes from some of their favorite giallo films. David wrote the film (entitled Final Cut) and acted as the director and cameraman, while Becker played the part of the killer, murdering homeless people the two drugged and abducted.
The BAU is called in after four of David and Becker's victims are found; during the course of the investigation, the two abduct, film and kill another homeless person (severely beating and torturing him beforehand) and also kidnap a former classmate, a girl named Kathy Bonders, who resembles the star of one of their favorite films, Il Morte Improvviso, or Sudden Death. David and Becker send a ransom note to Kathy's parents, demanding sixty-eight thousand dollars in exchange for Kathy's safe return. In actuality, the duo have no intention of letting Kathy live, and planned on simply murdering her before taking the money, which would help fund Final Cut. The day after dropping the ransom note off, David and Becker, to taunt the authorities, upload one of their murder videos onto the internet and leave another, near-identical ransom note (attached to Kathy's cell phone) at their old school while Gideon, Prentiss and a local detective are there questioning Kathy's friends and teachers. Later, with the ransom drop deadline approaching, the BAU manage to deduce who the unsubs are, their questioning of the film school staff and students having lead them to David and Becker. Going to the boys' old homes, the BAU find a picture of their trailer and track it down, only to find it empty. Hearing screams emanating from the woods, Morgan follows the noise and discovers Becker attacking Kathy with a knife, with David filming and giving orders. When Becker refuses to surrender, Morgan shoots him twice, killing him instantly. Seeing his partner go down, David drops his camera and rushes to Becker's body, hysterically professing his love for him before being arrested.
Brought to a police station, David is interviewed by detectives and denies any involvement in the murders, claiming Becker had forced him into helping him create Final Cut. After having Reid skim through the film script, Gideon talks to David, who tells him that Becker had intended to show Final Cut at festivals and other events, thinking it would make him famous. Playing off the fact that David was really the mastermind of the killings, Gideon tells him that Becker will indeed become famous - he will be a media sensation, having his face and name broadcast all over the news, with all the credit for creating Final Cut being attributed to him; David will just be another faceless victim. Realizing Gideon is right, David, unable to stand the thought of all the credit going to Becker, admits to his involvement in the murders, screaming "I wrote Final Cut! I directed it. Beck was just an actor-cattle, like Hitchcock said! I let the fucker act in my movie! He was Freddy Krueger, but I was Wes Craven!" After ranting, David states Becker always had trouble with executing his ideas, something which he has never had a problem with. This causes Gideon to state, before leaving, "Lucky for you. Because, David-execution is very likely where you're headed."
Originally, it was thought that only one unsub was committing the murders, but after evidence came to light that the slayings were being filmed, it became obvious that there were two killers. The unsubs were profiled as unassuming white males in their late teens or early twenties who would have juvenile records, fill every criteria of the McDonald Triad and likely, as the victims showed no signs of blitzing, use some ruse to lure them. The victims being drugged meant the unsubs lacked confidence in their abilities to overpower them unless they had the advantage, while the victims being groomed and dressed implicated the unsubs were acting out a fantasy of some kind and were socially immature, as the victims were all dressed as simple stereotypes that best suited their appearance. The murders were committed where the bodies were found, so the grooming likely took place somewhere else, where the unsubs would have more privacy.
As the unsubs continued to kill, they refined their skills and their confidence grew, resulting in the murders becoming more violent and frenzied. The unsubs tortured their victims before finishing them off, relishing in their fear and the feeling of power it gave them. As with all killing teams, there would be a dominant and a submissive; the killing would be apart of the dominant's fantasies and the submissive would merely be the tool used to help enact those fantasies. This may have lead to folie a deux, a rare psychological condition in which one person copies and incorporates the delusions and psychotic patterns of another person into their own personality.
Wandering through downtown Lawrence, the two, after spotting a vagrant who they found suitable for use in their film, would take a few pictures of them, approach them, and offer cash in exchange for doing a job they claimed they needed help with. If the victim accepted, David and Becker would take them to a restaurant or diner, where they would drug the food they bought with Rohypnol, to make the victim more pliable. The victim would then be taken to the boys' RV, where they would be groomed and clothed.
The victim would then be taken to a secluded area, where Becker, wearing a ski mask and all-black clothing, would chase them, terrorizing them and inflicting numerous non-fatal stab wounds, while David filmed and directed the scene. After some time, the victim would be finished off by having his or her throat slashed (with the exceptions of Frank Webster, who was stabbed in the heart, likely by accident, and his throat slashed post-mortem; and Roger Rondell, whose thigh was slashed). The body would be left at the scene and the footage they shot would be edited to resemble a murder scene from an obscure giallo film.
- Unspecified date: A victimless arson
- October 29: Elizabeth Hawkins (found in a soccer mom costume; her throat was slashed)
- November 14: Frank Webster (found in a business suit; fatally stabbed in the heart and his throat slashed post-mortem)
- March 9: Paula Creston (found in a waitress uniform; her throat was slashed)
- March 14: Roger Rondell (found in a business suit; his thigh was slashed)
- March 18: Benjamin Gray (found in a police uniform; his throat was slashed)
- March 19: Kathy Bonders (attempted, but survived; was non-fatally stabbed repeatedly; forced to wear a nun uniform)