"I wanted to have sex with my own mother. Don't you see how wrong that was? ... You knew what she was like! You should've protected me! A mother protects her child!"
Before Stuart was born, his mother Lynelle and her father Edward got into a car accident. Edward was killed and Lynelle suffered damage to her frontal lobe, which caused her to develop Klüver-Bucy syndrome. As a result, she became hypersexual, having sex with random men whenever she can. This got her pregnant with Stuart while she was still a teenager. After he was born, Stuart was adopted by his grandmother Gloria, since Lynelle could never be a proper mother to him. To explain the age difference between Stuart and Lynelle, Gloria told him that he was adopted and that Lynelle was his adoptive sister. She never remarried and had to deal with raising two children on her own. Stuart grew up believing Gloria's claim, and became aware of Lynelle's hypersexuality at some point. Eventually, he became infatuated with her, believing his feelings weren't morally wrong since they weren't biologically related. As an adult, he found a job at a construction company that built swimming pools.
In September 2016, Lynelle sought treatment for her condition and was prescribed medication, which helped keep her sexual urges under control. She took Stuart to a lovers' lane she used to frequent, causing him to believe she was going to have sex with him. When he tried to kiss her, she became disgusted, blurted out that she was his biological mother, and explained everything to him. Outraged by this, and horrified to realize that he had been in love with his own mother, Stuart snapped, killed Lynelle, and then buried her body in concrete. Because she often went missing intermittently to have sexual escapades, Gloria believed Lynelle was gone for the same reason and didn't report her missing. Meanwhile, Stuart, now with a damaged and confused love map, developed a delusion in which he believed all single mothers were hypersexual like Lynelle, and that he needed to literally turn their hypersexuality off. This caused him to abduct Sharon Landon (who he came across during his line of work), lobotomize her to damage her frontal lobe, and drown her in concrete when his experiment failed. He killed Lisa Barclay the same way a month later.
While working on Kim Conti's swimming pool, Stuart gets a phone call from Gloria asking about whether or not he has heard from Lynette. As he denies it, Kim and her son show up asking about the job, leaving when they are told by another worker that it will take a few more days. The conversation attracts Stuart's attention to Kim. He follows her to the local shopping mall, abducts her, and takes her to his house. After demanding to know when she will tell the truth, he comes at her with a power drill and fatally lobotomizes her. After dumping the body, he returns to work and spots a prospective victim, before leaving to clean his equipment and wash his hands. He visits Gloria and picks up a photograph, to which Gloria reminisces about his time playing on the swings. He claims that he doesn't remember, and Gloria remarks that she is worried about Lynette too. Stuart bitterly remarks about how he dodged the "crazy gene" by being adopted, and Gloria attempts to reassure him before telling him to change his clothes for dinner, remarking that he reeks of chlorine.
Later that night, Stuart returns to one of his dump sites and notices Prentiss and Alvez interrogating a teenage couple who they had mistaken for the unsub. He sneaks away, realizing the authorities are onto him. Returning home, he smashes the picture in a rage, attracting Gloria's attention. He rages at her, demanding that she tell him the truth about his adoption. When she doesn't and turns her back on him, Stuart knocks her out with a candlestick and abducts her. Tying her to a chair in the same lovers' lane where he killed Lynette, he begins digging a hole the next morning, just as she regains consciousness. She begs him to stop, but he demands her to tell the truth again. She claims that his biological mother couldn't take care of him, so she adopted him, but that doesn't placate him. All the while, Gloria struggles with her bonds and eventually breaks free, but Stuart catches up to her. He reveals that he knows the truth about Lynette, and that he killed her because she wouldn't sleep with him. Seconds later, Stuart dumps Gloria into the pit and pours concrete over her. Before Gloria could be fully submerged, the BAU arrive and Stuart flees. However, Alvez and Prentiss catch up with him and arrest him, while Rossi releases Gloria from the pit.
"When are you going to tell the truth?"
Stuart targeted single mothers in their mid-30s, with sons who reminded him of himself. He would find them through his job as a contractor. After spotting and selecting them, he would abduct them, take them to his workplace, restrain them to a chair, and stuff a drill up their nostril, pouring chlorine in it to damage their frontal lobes and therefore turn their perceived hypersexuality off. For the first two victims, they both survived these lobotomies, forcing him to stuff them into heavy drums and pour concrete into them, effectively drowning them. The third victim, however, was killed by the lobotomy and encased in concrete post-mortem. The drums would then be disposed at lovers' lanes, where he could revisit them over and over again.
When he attempted to kill Gloria, Stuart knocked her out with a candlestick, then tied her to a chair, took her to the same lovers' lane where he killed Lynelle, and dug out a hole. He then threw her into the hole, locked her in it with an iron gate, and began pouring concrete into it. He likely killed Lynelle the same way.
The unsub is a white male between the ages of 18 and 25 (initially believed to be in his 30s or 40s), performing crude chemical lobotomies on his victims. He drills up through the nasal cavity to insert chlorine into his victims' brains to destroy their frontal lobe, the area of the brain that controls impulse and inhibition. From this, he is likely trying to find power and control over his victims. His knowledge of the dump sites indicate that he is likely living or working in the area. He is also probably physically fit and drives a large van or pickup truck to transport the drums he keeps the bodies in. He doesn't seem to have any medical training, so when his experiments fail, he entombs his victims as a forensic countermeasure. The use of chlorine combined with his masonry skills suggest he is a tradesman of some sort.
Stuart is partially similar to Jeffrey Dahmer, a.k.a. "The Milwaukee Cannibal", and may have been somewhat inspired by him. Like Dahmer, he performed lobotomies on his victims to affect their frontal lobes and alter their sexuality (though Dahmer wanted to turn it on while Stuart wanted to turn it off), and when those lobotomies were unsuccessful, he disposed of the bodies in containers.
- Lynelle Barker (his mother; killed, presumably by drowning her in concrete)
- Sharon Landon (tortured with lobotomy and drowned in concrete)
- October 3: Lisa Barclay (tortured with lobotomy and drowned in concrete like the previous victim)
- October 10-11: Kim Conti (killed via lobotomy and entombed in concrete post-mortem)
- October 11: Unnamed victim (intended)
- October 12: Gloria Barker (his grandmother; attempted, but survived; was non-fatally bludgeoned with a candlestick, then tried to drown in concrete)
- Stuart shows some similarities with Anton Harris, a killer and abductor from Season Nine. Both lobotomized their victims, killed at least one victim by other methods, and were infatuated with their sisters (although the woman raised as Stuart's sister was actually his mother), whom they victimized.