"You should be honored that I chose your stories. I did everything that you described down to the smallest detail."
Mark was physically abused by his stepfather while he was a child, but when he managed to tell someone of what was happening and Social Services arrived, his mother changed the story and told them Mark had made up the abuse to protect her husband. This instilled severely misogynistic feelings in Mark, who came to believe that all women were liars. Mark later got a job working at an anonymous call center called Call First. While working there, he attended a lecture given by a psychiatrist, Dr. Montgomery Houston, on "document therapy", which involved patients writing out their violent fantasies and then having them admit their guilt or shame about having them in order to curb their impulses. On February 27, 2011, Mark snapped and killed Maria Bothwell in Tampa, Florida, cutting out her tongue to symbolize his belief that women were liars. He then killed two more women across the East Coast during the following year, also cutting out their tongues. He eventually settled down in St. Paul, Minnesota, and started a group that indulged in document therapy, intending to use his group members' stories as guidelines for his later murders. Among the members of this writing group was Peter Harper, whose stories were of a particular interest to him; he would use one of his stories as the basis for the murder of Patricia Moore.
In "The Gathering", Mark is first seen rescuing Nicole Francis from her drunken boyfriend, John Butler. When she thanks him, he refers to her by her name and then kills her. Later, he immediately goes after Laurel Tyson. Entering her house through an open window, he is surprised by Laurel's brother Daryl, whom he quickly kills as Laurel watches while hiding in a closet. Laurel then gets out of the closet and tries to escape, but he catches up to her and murders her. After waiting for a while, he ambushes Kayla Jefferson at a public laundry room, strangles her to death, and removes her tongue. The following night, Mark meets up with his group and reads their stories. Peter submits his late, and while reading it to the group, Mark stops midway, shocked by the content. After the meeting's conclusion, Peter confronts Mark about the murders, and he taunts him, saying that he should be grateful that he chose his stories as guidelines. Peter then tells him that the tongue removal wasn't part of his stories, closes the doors, and beats him to death.
Mark targeted Caucasian women of various ages. How Mark killed his first three victims or managed to find them is unclear, but it can be assumed that they had a strong online presence that was a factor.
In "The Gathering", he killed more frequently and found his victims through Peter's stories. His signature was gripping their tongues with pliers, removing them with a knife while they were still alive, and keeping said tongues as trophies. The only exceptions to this were Daryl Tyson, whose tongue wasn't removed since he wasn't the target, and Kayla Jefferson, whose tongue was removed post-mortem. He would then kill them by stabbing them to death with the same knife and leave their bodies where he killed them. An aspect of the latter phase of his murders would be identical to details given in the victims' blogs:
- Patricia Moore tweeted "saw one of the most beautiful oak trees in the park, wish I could just take a nap under it like I was a little girl", and her body was left under a tree in the park.
- Nicole Francis posted how she wanted to be rescued from a loveless relationship, and Mark saved her from her drunken boyfriend during one of his outbursts before killing her.
- Laurel Tyson had a blog where she described how much she loved S&M and how she wished someone would climb through her window and ravish her with bondage; Mark entered her house through an open window and she was the only victim that he bound around the wrists and ankles prior to the killing.
- Kayla Jefferson wasn't active online, but prior to the episode, she had a stalker who kept a blog about her, in which he stated "my fingers tightened around her throat. I could feel her heartbeat pulsing under my fingertips" after she repeatedly ignored him in real life; this blog inspired Mark to kill Kayla by strangling her with his bare hands instead of stabbing her.
The unsub is a fit, aggressive white male aged in his mid- to late-30s, who is practicing what is called vicarious fantasy assimilation, a term that is referring to the cathartic release that comes when he turns someone else's thoughts or fantasies into reality. He searches around the Internet and is selecting women with a strong online presence, all of whom frequently post their fantasies with descriptive detail. There are two levels to his M.O., the first being the enjoyment he gets from taking one of his victim's fantasies and turning it into a nightmare. Once the unsub establishes his dominance by subverting that fantasy, he moves on to the second level, which is removing the tongue. His M.O. is polymorphic, which means it has taken on different forms while his ritual never changes. It is likely that he has already targeted other women.
He learned about his latest victim from a post through an online community that exchanges psychopathic stories while remaining anonymous, which would explain the change in his M.O. All of his victims have been found on different platforms and the only common denominator is writing. He may have created his own writing group, using their stories and specific details in them to find the people his group wrote about and kill them himself. He might have attended one of Dr. Montgomery Houston's conferences or read about his form of therapy, and adapted it to fit his needs. However, instead of working on the steps, he is focused on one thing: acting out homicidal stories. In his mind, the unsub sees the tongues he takes from his victims as trophies, as they represent the telling of stories, and by taking them, he is able to claim possession of stories that are not his own. It is likely the members of his group are not aware of what he is doing, but because of the specific nature of the murders, it is only a matter of time before they finally realize it.
It is somewhat difficult to determine whether Mark is a serial or spree killer. While he targeted specific victims, had a specific M.O., and planned out the murders beforehand, it should be noted that he attacked several people within a span of roughly three days during the latter phase of his murders as depicted in "The Gathering".
Mark seems to have a basis in Russian serial killer Andrei Chikatilo. Both had parental figures who abused them (Chikatilo was abused by his mother, Mark by his stepfather) and had a similar M.O. of targeting women, cutting their tongues out, and stabbing them to death.
- February 27, 2011, Tampa, Florida: Maria Bothwell
- July 15, Atlanta, Georgia: Emily Hilton
- August 23, Charlotte, North Carolina: April Carver
- 2013, St. Paul, Minnesota:
- March 18: Patricia Moore (her body was left under a tree after her death)
- March 19:
- John Butler (Nicole Francis's boyfriend; assaulted while targeting her)
- Nicole Francis (killed in her car)
- March 19-20: The attack at Laurel Tyson's house:
- Daryl Tyson (Laurel Tyson's brother; incidental; his tongue wasn't removed)
- Laurel Tyson (bound around the wrists and ankles before being killed)
- March 20: Kayla Jefferson (was manually strangled instead of being stabbed; her tongue was removed post-mortem)
- c. March 21: Tammy (stalked for an unspecified amount of time and intended to drown)
- Mark seems to be based on a few unsubs in the show's past:
- Season Two serial killer Amber Canardo. Both were misogynists who were abused by a paternal figure (Amber by her father and brother, Mark by his stepfather), and when they denounced their abusers, the denunciations were withdrawn by their own mothers, resulting in their hatred towards women.
- Recurring serial killer George Foyet, a.k.a. "The Boston Reaper". Both were serial killers who were abused by a paternal figure as kids, with the maternal figure failing to protect them, had an M.O. that involved stabbing his female victims, performed overkill, committed their murders in multiple East Coast states, and were killed by being beaten to death in a fit of rage.
- Season Five serial killer Joe Belser. Both were misogynists who stalked their victims extensively, usually killed them by stabbing them, performed overkill, and committed a double murder in which they incidentally killed a male and also the female they targeted, respectively, in the homes their victims lived in.