"You should recognize the signs. It was your kind who did this to me!"
Prior to "The Fallen", Mills was a third-generation Santa Monica firefighter and presumably suffered from Hero Syndrome. He also suffered from severe mysophobia. He became well-known for his rescue of a total of six homeless people from an electrical fire at a warehouse; nine others were killed by the fire. In this rescue, he went into the burning building without wearing protective gear, hoping that this would help him, a third-generation firefighter, be easily seen as a hero. However, two of the people he saved were infected with a drug-resistant strain of tuberculosis, and he himself became infected through the unprotected contact. As a result, he was discharged from the fire department two months prior to "The Fallen" because of his illness. This worsened his mysophobia and also caused him to develop obsessive-compulsive disorder. He blamed the homeless people for preventing him from becoming "someone important" and viewed them as being a virus to the city. Eventually snapping, Mills started killing, claiming the lives of a homeless man on November 5, 2012, and another four days later. After killing them, he burned their bodies and dumped the remains at the local pier as a warning to the homeless community.
After claiming his third victim, Lindsay Leeman, Mills murders a homeless man named Jeremy the next day, this time by drowning him in bleach as a means of cleansing. At the same time, the BAU, called in by local police to investigate Mills's killings, receive help from Rossi's former sergeant from the Vietnam War, Harrison Scott, who is now a homeless man familiar with other vagrants living in the city. The following night, Mills approaches a homeless woman named Sarah Mapes and asks if she needs any help. She declines, just as Mills claims that he is trying to help her with "[her] problem" and then bitterly stating that she has a disease. He then holds her at knifepoint, but Scott, who is coincidentally nearby, confronts him. Mills cuts Scott's neck with the knife, but before he could continue, Sarah holds his arm down, allowing Scott, whose injury is superficial, to assault Mills. After a short fight, Mills is able to subdue Scott and make his escape with Sarah, nearly running Scott over with his car in the process. Surviving the attack, Scott relays what he saw to Rossi, including what he managed to catch from Mills's license plate, and this enables the BAU to narrow down and find Mills's residence.
At his home, Mills thoroughly cleans and tends to his wounds sustained from his fight with Scott. He then approaches a restrained Sarah and douses her with bleach, but he overhears the BAU entering his property and flees. As the team manages to rescue Sarah, he is able to escape and carjack a vehicle. Being familiar with the homeless population, Mills is able to easily find Scott and abducts him from an alleyway, angrily stating, "You've ruined everything." Taking him to the same warehouse where he rescued the six vagrants, Mills aggressively douses Scott, then the floor with gasoline. When Scott taunts him, he slashes him in the chest with a knife, but begins to cough. Scott taunts him again, prompting Mills to activate a lighter, snarling that he was going to matter. Then, Rossi, JJ, and Morgan burst in and attempt to convince him to put away the lighter. Mills refuses, and Scott orders Rossi to back down. He obliges and leaves, followed by JJ and Morgan, but the three sneak back into the warehouse as Scott distracts Mills. JJ and Morgan hold him at gunpoint while Rossi unties Scott. The four try to appeal to Mills, but they are unsuccessful and Mills, standing on the pool of gasoline, sets himself on fire and dies.
Targeting homeless people of both genders and different races and ages, Mills would lure his victims into his car at night with some sort of ruse. He killed his first three victims by stabbing them in the chest or abdomen with a knife, took their bodies to an unspecified location, and burned them with unknown chemicals as a form of sterilization. He then dumped the charred bodies by the local pier prior to the local rush hour, hoping they would be seen by other homeless people.
Later, he killed his fourth victim and attempted to kill his fifth victim by drowning them in bleach, shaving their hair and handcuffing them to the metal container they would be drowned in beforehand. The body of his fourth victim, unlike the others, was left near a homeless shelter and not by the pier. His fifth victim also had the remnants of her hair washed with Quell (a medical-grade disinfectant). During his first attempt on Harrison Scott's life, he gave him a superficial cut in the neck with the same knife, tried to manually strangle him to death, and then tried to run him over with his car. During his second attempt to kill Scott, he tied him to a chair, doused him with gasoline, tortured him by slashing him with the knife, and intended to burn him alive.
"I was going to matter!"
The unsub is a white male who is hunting the homeless, always strikes at night, and is hoping to make a social or political statement through his murders. He has evolved from destroying his victims with fire to wanting them to be easily identified while still depersonalizing them. He is specifically burning his later victims with bleach in an effort to scare the homeless. Shaving the heads of his victims is yet another way of humiliating them. This unsub has a method of transportation and the facility to bleach and burn his victims. He dumps the bodies prior to the morning commute, so he could have a daytime job. It would be hard to detect him, as he blends in with the community, and because of this detail, he is probably a local who feels wronged by the homeless community. He does not draw attention to himself and he is meticulous in his planning. The fact that he is now dumping his victims away from the pier means he is aware of the ongoing investigation and the killings are far from over.
His latest murder indicates the unsub has changed his M.O. to perfect his message and is now torturing his victims because he enjoys killing them. As he improves his methodology, he is also improving on what gets him off. The usage of bleach means he wants to cleanse the area of the homeless community in the socioeconomic sense. He lives within two miles of his comfort zone and drives a Ford Mustang registered to himself, since it is doubtful he can maintain a healthy relationship. Burning his first three victims post-mortem, the usage of bleach in his fourth murder, and putting Quell in Sarah Mapes' hair all relate to forms of sterilization. If he is indeed cleansing the streets, he could be treating the homeless population like an infection, seeing them as a virus and himself as the cure.
Chad seems to be based on Israeli serial killer Nicolai Bonner, a.k.a. "The Homeless Murderer". Like Chad, Bonner targeted homeless people, killed his victims by stabbing them, and burned his victims post-mortem. Both also were affected by tuberculosis in some way (Chad suffered it himself, while Bonner's wife died from it) and were motivated by their hatred of the homeless community. Chad's first three murders also seem to have been based from the M.O. of serial killer Ali Reza Khoshruy Kuran Kordiyeh, a.k.a. "The Tehran Vampire". Just like Chad, Kordiyeh lured his victims into his vehicle, stabbed them to death, and burned their bodies with an accelerant.
- November 5: Unnamed homeless man (stabbed and burned post-mortem)
- November 9: Unnamed homeless man (stabbed and burned post-mortem like the previous victim)
- November 11: Lindsay Leeman (stabbed at least three times in the heart and burned post-mortem)
- November 12: Jeremy (shaved his head and drowned in bleach; left his body near a homeless shelter)
- November 13: The alley abduction and attack:
- Harrison Scott (incidental; attempted, but survived; slashed in the neck, assaulted, non-fatally manually strangled, and tried to run over with a car while abducting Sarah Mapes)
- Sarah Mapes (abducted, taken to his home, and attempted to drown in bleach; was rescued the next day)
- November 14: Harrison Scott (abducted, taken to an abandoned warehouse, tied up, slashed in the chest, and attempted to burn; was rescued)
- It should be noted that although he sets fires, Mills is not an arsonist; he is a serial killer who uses fire as a weapon. Vincent Stiles and Kaman Scott were described the same way.
- Mills seems to be based on a few unsubs in the show's past:
- He is mostly similar to Season Two serial killer Ronald Weems. Both were "House Cleaners" who held an intense hatred towards their victimologies (in Chad's case, it was the homeless, for Ronald, it was prostitutes), would kill their victims by stabbing them, killed their victims at a certain time of day (Chad would kill at night, Ronald killed in the morning), and deviated from their original M.O.'s in the case of their last murders.
- Mills's hatred of vagrants, his treating them as if they were contagious diseases, and the BAU's formal classification as a "House Cleaner" is similar to the attributes of serial killer Charles Holcombe, who appeared late in Season Two. Also, the outfit Mills was wearing when he killed his fourth victim is similar to the one Charles would wear when he killed his victims.
- Also, his obsessive-compulsive disorder, tendency to wrap everything in his car in sheets of plastic, and killing his victims by stabbing them is reminiscent of Season Four serial killer Vincent Rowlings.