|“||It's been a long time, Ellen. It's been a long time.||”|
— Cochran to Ellen
William was a teacher at the Glenbury College of Art and Design, where he met and mentored a student named Ellen Clark, who was briefly auditing courses at the school. The two fell in love and had a son named Charles. William was later fired from his job after having an altercation with a colleague and ordered to take a mandatory psychiatric evaluation, indicating an underlying mental illness at the time. In 2007 at Nokomis, Florida, Charles wandered away from Ellen while she was painting and drowned in a nearby lake. Following the tragedy, William split Charles's blanket with Ellen and kept his half in memory of his son, with Ellen doing the same. However, he was still traumatized by the loss and began drinking excessively. Eventually, he temporarily became homeless and was arrested at least once for disorderly conduct. In an attempt to clean up his act and get his life back together, William moved to Georgia and became a handyman. However, this fell apart when he realized Ellen used her half of Charlie's blanket in one of her pieces of graffiti art in Detroit. Believing this to be an insult to Charlie's memory, he cleared out his bank account and purchased a one-way ticket to Detroit, intent on seeking revenge against Ellen for what she did, but not before destroying the reputation of her alter-ego, "Morpheus".
The Night Watch
After setting up the murder scene of Russell Pearson, William signs it with Morpheus' name and flees the scene. The next day, he abducts an eight-month-old infant, Corrine Wallace, from her crib, spraying the baby monitor with paint and again leaving on the wall of her nursery Morpehus' signature. Another day goes by and he is seen painting several roses black while the baby watches, until Corinne starts crying; William gently picks her up and tries to soothe her. He then abducts and kills Markus Ivey, one of Ellen's rivals in Detroit who William previously tipped off regarding the location of one of her street artworks. Afterwards, William sets Markus' body up in a gigantic baby mobile over an empty cradle and leaving him for the BAU to find. He goes to find and kill Corey Marlin, torturing him for Ellen's location beforehand. William returns to his hideout to find Corinne crying, giving her a toy and assuring her that it's "almost over now". Hours later, he abducts Ellen, taking her to the location where he is holding Corinne. William accuses her of not caring about Charles, demanding to know why she used her baby blanket in the art piece. When Ellen replies that she did it to honor a homeless child who died, he becomes enraged at what he claims to be her strange logic and tells her that she has to choose between dying herself or letting Corinne die. She chooses herself, and William forces her up to the roof at gunpoint. Ellen is trying to talk William down when Hotch arrives and William takes her at gunpoint, apologizing to her before pulling them both off into his last art piece, resulting in their deaths.
William initially targeted people who were directly or indirectly related to the controversial development project in downtown Detroit, but later switched to other victims as he began to reach his endgame. His first victim, Russell Pearson, had his mouth covered and his arms tied together with duct tape; was hung upside-down by his feet with rope; and killed by a large, elaborate spring-loaded mousetrap. Afterwards, he left a piece of Morpheus-based graffiti art beside the crime scene to point investigators to Ellen. His second victim, Markus Ivey, was manually strangled and his body was then strapped to a gingerbread man prop that was part of a giant baby mobile prop for another Morpheus-based graffiti art piece. His third victim, Corey Marlin, was beaten, bound with duct tape, and tortured via beating with an unspecified object to find out where Ellen was located before being shot and killed with his own handgun. Ellen, his final victim, was killed in a murder-suicide in which the two of them fell off a rooftop onto a piece of graffiti art announcing the death of Morpheus.
No profile of William was made by the BAU, as they were focused on Morpheus as a suspect.
- 2015, Michigan:
- October 24, Detroit: Russell Pearson (hung by his feet and killed by a large mousetrap)
- October 25, Grosse Point: Corinne Wallace (abducted and held at gunpoint; was rescued two days later)
- William is similar to Season Six criminal Hollis Walker. Both were serial killers who had stressors involving the death of a family member (William's son died in a drowning accident; Hollis's mother committed suicide), had academic professions (although William was fired from his job), had a varying M.O. (one of which was shooting), and tried to frame someone for their murders.
- William is also similar to Season Eight criminal Tess Mynock. Both were delusional killers who lost their sons in the exact same manner (drowning in a lake while left unattended), which caused them to break up with their romantic partners. They were also motivated to commit their killings because of their sons' deaths, and later committed suicide.
- According to his report report, Cochran's Social Security number is 964-00-1912.