This list covers all minor criminals mentioned in Season Eight.
Unnamed Seattle serial killer
An unnamed serial killer on whose case Alex Blake worked; it was her first case with the BAU since her leave of absence following the Amerithrax case. In The Silencer, she mentions that he used his own child to bait his victims (not unlike Season Three criminal Joe Smith) and made Gary Ridgway (who also used his own child to lure in victims, though not directly, and who was also active in Seattle) "look like a saint". The case was mentioned again in The Replicator, where it is revealed that a woman had been killed by a copycat.
A serial killer and inmate who was put on a work detail with future serial killer David Roy Turner, whom he tutored on how to kill without getting caught. He committed the murders of four prostitutes in New York, biting and fatally bludgeoning them with a hammer, before being arrested and convicted in 1989. Following David's release, the two maintained close contact until Rudy died from a massive stroke in July 2011 while still in prison.
Unnamed Whitewood assailant
An unidentified man who was responsible for abducting future killer Toby Whitewood and his sister in 2010, sexually assaulting the latter and then killing her in front of Toby before escaping. This act would be responsible for inducing Toby's psychopathic tendencies. It is unknown if the man was apprehended afterward.
An unidentified man who killed a woman and her daughter while stealing their vehicle in Seattle. Local authorities managed to catch up with him and arrest him, to which he confessed. A misinterpretation about the man's identity would be one of the stressors behind Carl Finster's crimes.
An unnamed man who robbed and then fatally shot Alex Rain in front of his son Adam's eyes. It is unknown if the robber was caught and convicted of his crime.
A street artist who was wanted by the San Francisco Police Department. Bryan Hughes left the body of one of his victims close to one of his artworks. It is unknown if he was caught after the BAU's investigation into Bryan's murders.
Prior to the events of the episode, several conversion camps were shut down for using brutal and illegal methods. JJ mentioned that one camp was shut down the previous year for using electroshock therapy, while Blake mentioned another had stripped its attendees naked and screamed profanities at them before being caught.
A conversion camp that used brutal and illegal methods, such as electroshock therapy, on their patients. Their actions become the catalyst for future serial killer Paul Westin. By the end of the episode, the camp is raided and the employees are arrested.
A prostitute who abused and molested Paul Westin at Camp Willing. She is currently incarcerated for her crimes and proved to be of help for the case at hand.
Jack Lee Kemper
A serial killer who was investigated by Gideon. He targeted nurses, killing four, and was apprehended before he could kill a fifth victim. Prior to his arrest, Donnie Bidwell was considered a primary suspect. He died of an overdose five months prior to the episode, the same time the Replicator first committed murder. Kemper's surname is taken from real-life serial killer Edmund Kemper, presumably as a reference to him.
Unnamed cop killer
An unidentified man who fatally shot Detective David Rizzo's partner in 2008. It is unknown if the killer was connected to the federal case Rizzo and his partner were investigating at the time.
A man who was one of pedophilic serial killer Carl Buford's molestation victims. As an adult, he was arrested multiple times for domestic violence and possession of methamphetamine, and once for aggravated assault. He was a suspect in Rodney Harris's spree killings and was interrogated by Morgan, but Morgan eventually realized, through Keron, that Rodney was the killer.
The boss of Sean Hotchner, who would date-rape women at the bar he owned using ecstasy-tainted wine and later became indirectly responsible for some of the deaths of victims of overdosing deaths. He is arrested when he attempts to stab Sean, who is wearing a wire while trying to wrangle information out of him concerning a case. Thane is later put under witness protection when it turns out that the unsub could be potentially targeting employees of the bar (including him). After the unsub is arrested, it is likely Thane was arrested for his actions.
An accomplice of Thane Parks in his date rapes and later the tainting of wine with ecstasy. She is put under witness protection alongside Thane and presumably arrested after the unsub is arrested.