|“||We're gonna live forever.||”|
— William Pratt
In 2013, William lost both of his parents in a car accident, and he went to live with his aunt. As a result of the loss, he became shy and withdrawn. He befriended a fellow student and amateur writer named Connor Holt, whom he helped create "The Mirror Man", an entity in creepypasta. The lore of The Mirror Man described it as a murderous entity appearing in windows or mirrors behind people as they post photos of themselves (or selfies). When the tale of The Mirror Man became extremely popular on the Internet, William, looking up to Connor as a role model and wanting to please him, decided to give the fictional entity more popularity through a series of serial killings based on it. He decided to target Tara Harris, a teenage girl moonlighting as a famous online celebrity, as his first victim. This choice was likely made because he was jealous or agitated by her popularity, which centered around her increasingly raunchy selfies and rebellious attitude towards her parents.
Eventually, on the night of November 14, 2014, William kills Tara by bashing her head against a mirror, then stabbing her and slashing her throat with a shard of the same mirror. Afterwards, he poses a selfie of Tara and posts it online with the "fear me" hashtag. The attack draws the attention of the BAU, who all become worried that the massive amount of public attention revolving around this murder will provoke the unsub to strike again. Whilst enjoying the media circus around him for the entire day, William goes to the local shopping mall with Connor and Daniel Osbourne, discussing The Mirror Man lore with them. The trio then spots Alexander Chase, and William learns that he and Tara made a challenge video together. The following night, he ambushes Alexander as he is seated in his convertible, repeatedly shooting him in the back with a nail-gun, making sure that the pattern resembles a hashtag symbol in the back of his seat, then executes him with a shot to the neck. He then poses another selfie and names himself 'Mirror Man' in the newest message, not liking the title of 'Selfie Killer' that the media gave him. William then watches a video of Daniel mocking The Mirror Man and trashes his computer in a fit of rage. Going to Daniel's house the following night, he ambushes and kills him much more brutally than the previous murders. The amount of overkill present at the crime scene and the motivation lead investigators to reconsider the unsub's age from that of a 30-40-year old man to that of a teenager. Conner himself is briefly accused of William's murders, but after questioning, he reveals his suspicions about him.
When Connor is brought in for questioning by the BAU, William bikes past his house and sees the news cameras discussing The Mirror Man, pleasing him. He then arrives at the shopping mall to attend to his job at the kiosk, bringing the nail-gun with him in an apparent attempt to return it. As he locks his bike, he notices a government-issued SUV containing Morgan, JJ, and Kate driving up, the BAU having identified him as the unsub with Connor.
Panicking, he flees into the parking garage, pulls out the nail-gun, and takes a shopper, Kimmy, hostage with it. Morgan, JJ, and Kate hold him at gunpoint, and William demands to see Connor. He is nearly talked down by JJ, but a news van drives up, and the news crew records the standoff live, essentially giving him an audience. Emboldened by this, William turns to the camera's direction and shoots Kimmy in the neck, getting shot in the back by Morgan in return on national TV. He is taken to the hospital, along with Kimmy. Despite losing a lot of blood and having some nerve damage, he survives his gunshot wound, while Kimmy is also saved in time. Connor visits William and the latter remarks that they and The Mirror Man are going to be famous. He adds that he will be released from jail soon, being a minor. All the while, Connor expresses shock at his attitude. William finally asks Connor for his cell phone; when Connor gives it to him, he takes a selfie, saying, "No one will ever believe I got shot." What exactly becomes of William Pratt is unknown, but since he survived the gunshot wound, it's safe to presume that he is placed in a mental institution afterward.
William found all of his victims through the local shopping mall where he worked and killed them at night. During the murders, he would wear a dark hooded sweatshirt, dark pants, and leather gloves to effectively conceal his identity and avoid leaving behind any forensic evidence.
He targeted his first victim, Tara Harris, for her status as a popular online celebrity. He broke into her home and intended to kill her by shooting her with a nail-gun as a reference to The Mirror Man's clawed hands. Instead, he bashed her head against a mirror, then slashed her in the abdomen and killed her by slashing her throat with a shard of the mirror. This was provoked after he saw her looking at herself in the mirror and wanted to act on the irony of her being vain. He then targeted his second victim, Alexander Chase, due to his connection with Tara. He killed him by shooting him through the neck from behind with the nail-gun while Alexander was seated in his convertible. His third victim, Daniel Osbourne (who was a former fellow student and one of William's friends), was targeted after the latter posted a video on Twitter challenging and insulting him. He shot him repeatedly in the hand, forced his way inside his home, and shot him again in the rear leg with the nail-gun before torturing him by stepping on the areas where the nails hit; he then killed him by shooting him repeatedly in the face and then mutilating his wounds post-mortem.
Prior to killing his victims, he would alter their selfies he got from their online accounts by adding The Mirror Man behind them using Photoshop. William would then tag them with the altered selfies after hacking into their social media accounts by finding their passwords. He would do the same with dozens of other random people worldwide. For the first two murders, William would pose selfies with the victims' bodies and post them online with the message "Fear me"; the message for the second selfie also included "#MirrorMan" due to his dissatisfaction with being called "The Selfie Killer" at the time. Also, during the second murder, he fired his nail-gun repeatedly at the back of the driver's seat of Alexander's car, forming a hashtag with the holes as a calling card.
The unsub was originally profiled as an older man, aged in his late 30s to early 40s, based on the organization and sophistication put into the killings. He is a narcissistic sadist who is blitz-attacking his victims with weapons of opportunity. He sees these weapons as symbols of his victims' vanity. For the first victim, he used a mirror, then nails made in the form of a hashtag for the second victim. A hashtag is a popular method of topic-tagging on social media websites; these murders are essentially the unsub's way of making an online statement. He is trying to make a name for himself by building off from the Internet legend of The Mirror Man. He is becoming immortal as the stories and legends grow, fueling him to only act again. He is probably not socially adept in his personal life and is most likely subservient to his peers, although he does not view it that way. His need for attention probably stems from abandonment in early childhood.
He is also technologically proficient, as he was to alter a twisted image of Mark Twain onto his victims' photos. His usage of a distortion of Mark Twain as the face of The Mirror Man is very important to him, as Twain was described as the father of American literature, and this unsub sees himself similarly; he wants to become the forefather of modern lore. He will be monitoring web traffic, media websites, and chat-rooms as he enjoys the panic and hysteria created by his crimes. The victimology seems personal, which suggests that the next victim will be someone connected to the first victim. His endgame will involve fame and/or fear in some way.
The unsub was later revealed to be a teenager due to the impulsiveness and rage that was evident in the third murder and the signs of overkill, which were not present in the previous murders. His latest kill points to a devolution in his psyche and he is no longer killing people to show how great he is, but killing the people who insult him. He has decided to literally bring the tale of The Mirror Man to life. He would look up to Connor Holt, the creator of The Mirror Man, and even be younger than him. He is quiet and clings to his friends because they are all he has. He likely suffered a recent abandonment or loss of a loved one. He may have also aided in the development of The Mirror Man, judging by the two completely different types of handwriting found on personal creator's notes on the legend.
It is difficult to determine whether William is a serial or spree killer. While he targeted specific victims, planned out most of the murders beforehand, and had a specific M.O., it should be noted that he attacked his victims almost daily and was disorganized during the actual murders, which is uncharacteristic for usual serial killers.
William's murders seem to be based on a series of attacks, all of which were isolated incidents and allegedly inspired by the Slender Man, a fictional character that originated from creepypasta. Like the perpetrators of the incidents, William attacked his victims in a manner similar to what was described in the lore of a creepypasta character; however, unlike the real-life perpetrators, William was successful in actually killing his victims. The most notable of these real-life incidents occurred in Waukesha County, Wisconsin, and was perpetrated by two twelve-year-old girls, identified as Morgan Geyser and Anissa Weier; the incident was compared to William by the BAU.
William's act of posting online images of his victims after he killed them, accompanied with taunting messages, seems to be almost identical to a murder in Port Orchard, Washington, which occurred a week prior to the airing of "Hashtag". Like William, the suspected perpetrator, David Kalac, posted photos of his victim online after killing her, at the controversial imageboard website 4chan, along with a few messages.
- November 14: Tara Harris (bashed her head against a mirror, then slashed her once in the abdomen and fatally slashed her throat with a piece of the same mirror)
- November 15: Alexander Chase (fatally shot once through the neck from behind and 56 times in the back with a nail-gun post-mortem)
- November 16: Daniel Osbourne (shot three times through the hand and four times in the rear leg with the nail-gun, tortured, fatally shot at least once in the face with the nail-gun, and mutilated his face post-mortem)
- November 17: Kimmy (incidental; attempted, but barely survived; was shot twice in the neck with a nail-gun)
- William shares some similarities to Owen Porter. Both were serial killers who were underage, lonely, disorganized in their killings yet specifically targeted their victims, were motivated in some way by their role models, older boys who went to the same schools as them and befriended them when no one else did, killed their last victims in their homes, and performed overkill on them. Also, both killed their victims by shooting them with some type of ranged weapon or stabbing them with an improvised object.
- William is also somewhat similar to Robert Johnson in the sense that both were serial killers who stalked their victims online, were motivated by a need of attention, killed their victims in their homes at night, and deviated from their original M.O.s in the case of their last murders.