Onario was born to siblings Dominico and Renada Scarpa, the result of a rape committed by Dominico upon Renada. Before Onario's birth, Scarpa was prosecuted for the sexual assault, but Renada, having been painted as a drug-addicted, mentally unstable woman by the defense, fled to Naples before the end of the trial, in which Scarpa was acquitted. At Naples, in the end of 1994, Renada attempted to self-induce a late-term abortion, but it failed and she died in the process, resulting in Onario's birth. He was then left in the care of his grandmother Elisa, who named him Onario Alighieri. The surname was the same as Italian poet Dante Alighieri, and was meant as a cruel joke. Elisa was abusive towards Onario, who suffered from developmental challenges due to being a product of incest.
When he was fifteen, Onario, having already developed a propensity for violence, drowned a female classmate and was sentenced to a youth detention facility located outside Rome. He stayed there until his twenty-first birthday, after which he was immediately discharged in 2016. Hoping to understand his violent, murderous impulses, Onario sought out Elisa, who revealed the existence of his father to him, and the fact that he was a prime suspect in the case of the Monster of Florence, an uncaught Italian serial killer who murdered a total of seven couples. Onario subsequently developed a twisted sense of self and tried to reach out to Scarpa the following year. Scarpa initially shunned him, but then realized this was an opportunity to ensure the Monster lived on even after his death. As a result, he instructed Onario to carry out a series of murders using his M.O.
One night, Onario sets his sights on Peyton Moss and Gina Price when he approaches them from his car. Onario gets out of the car, approaches them and shoots Peyton. Gina gets out of the car and flees, but Onario shoots her and she falls. Gina begs Onario not to kill her, but Onario ignores her and kills her. Afterwards, he takes Gina's phone and calls her sister, Alison. By this time, Alison has gone to the police and tells Commissario Galterio Conde about her sister. Alison and Galterio answer Alison's phone, who they think it is Gina, but it was a recording of Onario's murder of Gina. Afterwards, Onario leaves the box of Gina's genitals and Gina's phone by the fountain, where the police find them. The next night, Onario targets a couple named Rory Poole and Diane Roberts. He ambushes them and kills them. Afterwards, he uses his surgical equipment to remove Diane's genitals.
The following night, Onario is seen dancing while holding knives and throwing papers on the floor while an opera is playing on the radio. Then, he folds a paper and puts it in an envelope. Later, the IRT identify Onario as the unsub and Scarpa is involved, including killing people in the past. The team splits up, and Seger goes to Carmela Tafani's residence where she finds out that Scarpa has abducted her and is about to kill her. When Seger tells Scarpa to let go of Carmela, Onario ambushes her from behind and tells her to drop the gun, which she does. While both men hold the two women hostage, they both try to reason with Scarpa and Onario, but Onario tells them that no one is going anywhere. Then, Onario tells Scarpa that he brought both women to him, who wants to punish them and make them suffer. As Onario is about to shoot Seger under Scarpa's order, she tells him that Scarpa is not listening and that Onario did all the murders for his father and he still doesn't see him for what he is. Seger continues saying that he brought both of them to show them what he really is. At first, Onario denies it, but he starts to believe what Seger is saying. He holds his father at gunpoint and is about to kill him, when Seger takes out a gun from her ankle holster and shoots Onario twice, killing him instantly. Then, the team shows up and arrests Scarpa afterward.
In order to give off the impression that the actual Monster had returned, Onario copied Scarpa's M.O. perfectly, targeting couples making out in public properties and shooting them with the very .22 Beretta his father used in the original murders. He would first shoot the males in a sudden ambush, then the females while they were attempting to escape. After killing the female victims, he would mutilate their genitalia with Scarpa's own surgical blades and wear surgical gloves to avoid leaving fingerprints. All of the victims were tourists from either the U.S. and the U.K.; though this appeared to be a coincidence, it could also be a reference to the fact that some of Scarpa's victims were tourists from other Western countries.
When he killed his first victim, he drowned her in an unspecified manner. During the standoff at Carmela Tafani's home, he used a .22-caliber Beretta U22 Neos pistol.
The IRT briefly profiled Onario as an abused and emotionally abandoned boy who was tormented by innate impulses to do harm onto others. He doesn't understand these impulses and certainly cannot control them, and believes there is an explanation for them in his family's history. After finding his father, he developed a twisted sense of self and believed that the Monster of Florence was part of his birthright.
Onario may have been partially inspired by speculation that Pietro Pacciani, a suspect in the Monster of Florence case, needed the assistance of an accomplice for the last Monster murders, due to Pacciani being in poor health at the time. He may have also been inspired by Giancarlo Lotti's claim that a "Florentine doctor" (like Scarpa) had hired him and Pacciani to collect body parts from women.
The idea of the Monster returning in the modern day might have been inspired from Riccardo Viti, a.k.a. "The Monster of Ugnano", another Florentine serial offender who was speculated by the press to be the Monster returned after he tortured, raped, and "crucified" a Romanian prostitute in 2014. Much like Viti, Onario also posed his female victims like they were crucified, which is something that the Monster didn't do.
Outside of the Monster, Onario may have also been partially inspired by Heriberto Seda, a well-known copycat of the Zodiac Killer. Both were deeply fascinated by the original killers they copied, copied the M.O.s of said original killers as much as they could, attempted to give the impression that they were the original killers, and met their ends during standoffs inside a female's home.
In addition, a scene in which Onario called police to taunt them, using the cellphone of a victim's sister, seems to have been indirectly inspired by the Gilgo Beach Killer, who taunted the sister of a victim after calling her.
- Unspecified date in 2009: Unnamed female classmate (drowned)
- October 21: Peyton Moss and Gina Price:
- Peyton Moss (shot in the chest)
- Gina Price (shot twice in the back and chest; mutilated her genitals and removed her reproductive organs post-mortem)
- October 22: Rory Poole and Diane Roberts:
- Rory Poole (shot twice in the chest)
- Diane Roberts (shot twice in the back and chest; mutilated her genitals post-mortem)
- October 23: The Tafani home standoff:
- October 21: Peyton Moss and Gina Price: