"I need to do this. I can't stop it."
Walter started killing in Philadelphia in 1986 and garnered the nickname The Keystone Killer. It is mentioned that he served in the Reserve Officers' Training Corps in high school, got a degree in criminology from Villanova at some point, and served in the U.S. Air Force for four years. He then held down several short-lived jobs, including a county worker and a claims adjuster. During his initial serial killing phase, he was an alarm installer; all of his jobs gave him easy access to peoples' homes. At some point, he worked together with Scott Harbin, a fetish burglar who was suspected of being the Keystone Killer. He selected all of his intended victims before he started killing them, apparently planning to kill them in that specific order; all were brunette women in their twenties at the time. When Walter was about to kill Carla Bromwell in 1988, he was involved in a car accident on September 21, which severely damaged his spinal cord. He recovered, but lost a lot of strength in his right side. During his recovery period, he apparently suffered from depression, which went away when he started killing again. At some point in his life, he got married.
Starting with Carla Bromwell, whom he targeted because he is compelled to finish the job with her, he begins killing again in 2006, his M.O. having been drastically changed as a result of the car accident. After killing Carla Bromwell in "Unfinished Business", he sends a letter to Max Ryan, the former profiler who chased him in the 80s and went on to write a book about The Keystone Killer. After investigating Carla Bromwell's murder and being led to Scott Harbin via a puzzle sent along with the letter provides the clue, Max, who is temporarily employed as a consultant, and the BAU realizes that Walter changed his M.O. because he had to. Narrowing the list of suspects down, they find Walter and arrest him just as he is about to kill another woman named Sylvia Gooden, whom he has been stalking since the 80s. As he is arrested by Morgan, Walter taunts Max.
He would go on to be mentioned in Pay It Forward as an example of serial killers who go dormant for long periods of time or completely quit.
Max's original profile of the unsub described him as someone in his late 20s who would have a possible military background, drive a late-model American-made sedan, and live in Philadelphia for his entire life. He would also possibly hold some position of authority at work.
The more recent profile of the unsub described him as a white male in his late 40s. He is a narcissistic sociopath who kills because he wants to, rather than a psychopath who kills because he has a compulsion to do so. He is apparently also a perfectionist, having to complete the series of killings he started. Much has changed in the last eighteen years; the Keystone Killer has gotten older as well as his victims. Most unsubs have specific fantasies, as if they are killing the same person over and over. Originally, he had a preference to young brunettes, but is now moving on to older women. It is possible that he could be devolving into a frenzy; it is only a theory, but if he is in a frenzy, there is no telling how much he will fall apart or how many people he will kill. As the killer's puzzles contained references to even the smallest details of the crime scene, it means that he was probably taking pictures of the places he killed at, using the photographs to relive the murders. The Keystone Killer's change in M.O., which included blitzing his victims, meant that he may have been partially incapacitated in some way, either due to a stroke or (more likely) an injury.
Walter targeted brunette Caucasian women in their twenties and selected a number of them beforehand. During the original 1980s killings, he used his various occupations to gain access to his victims' homes without force and also apparently stalked his victims for some time prior to killing them. His original signature was tying his victims up using an intricate knot. He then killed them by strangling them manually. When he resumed killing, he continued targeting the same women he had chosen to kill in the 1980s. Because he had lost a lot of strength in the right side of his body due to the car accident, he had to use flex cuffs to restrain them, which he forced the victims to put on themselves at gunpoint, pistol-whipping them to incapacitate them. He then placed a plastic bag over their heads, suffocating them.
He would write taunting letters to the police, like Son of Sam and the Zodiac Killer, as well as word search puzzles which contained keywords relating to the crime scenes. He also took photos of his victims, before and after they were killed, developed them himself, and then collected them in a scrapbook. He would also put other miscellaneous items from their crime scenes into the scrapbook. He also took their driver's licenses and newspaper articles about his crimes as trophies.
Walter has several similarities to the Zodiac Killer and Dennis Rader, the BTK Killer. Like him and the Zodiac, he wrote taunting letters containing some sort of puzzle to the police (Zodiac sent ciphers; Walter and BTK sent word search puzzles). Like BTK, Walter worked as an alarm installer until 1988 and was in the U.S. Air Force for some time. Also, they both strangled or suffocated their victims, or at least most of them, took their driver's licenses as trophies, and stopped killing in the 80s only to return in the present.
- Unspecified date in 1986: Abby Tolbert
- Unspecified dates from 1986 to 1987:
- Amber Binder
- Cynthia Fullerton
- Victoria Fitzgerald
- Gloria Devlin
- Unnamed victim
- Unspecified date in 1987: Amy Jennings
- September 21, 1988: Carla Bromwell (stalked for an unspecified amount of time and intended to kill)
- February 25: Carla Bromwell (suffocated with a plastic bag instead of being manually strangled)
- February 27: Sylvia Gooden (attempted, but survived; stalked for an unspecified amount of time and was non-fatally suffocated with a plastic bag)
- Season One
- Season Four
- Season Eight
- Pay It Forward (mentioned)