"This is proof! This is proof that I can protect our new baby! This is proof...proof that I'm never, NEVER going to let anything or anyone ever hurt him!"
Tanner worked as a freelance photographer and also a night-time doorman. At some point in his life, he married a woman named Hannah, with whom he had a son named Jeremy. One day, in 2012, while ten-year-old Jeremy was playing wrestling with a friend, Tanner was called away and left the room for a minute. When he returned, he found the friend trying to resuscitate Jeremy after a choke-hold had gone wrong. Jeremy died after being on life-support for a few days. Consumed with guilt over failing to watch his son, Tanner began watching the tenants of an apartment building that was next to his workplace, paternally transferring to them and creating a fantasy where he was their guardian. On September 13, 2013, Hannah, blaming him for Jeremy's death, divorced Tanner and threw him out of the house, telling him that he was too weak to protect Jeremy. Tanner, becoming desperate to prove to Hannah (who became pregnant with another man's baby) that he was not weak and that he could protect people, snapped and began killing perceived "bad influences" to the tenants of the apartment. This delusion drove him to murder Michael Dunsmore, who was a marijuana user, and Ryan Maxford, who was always broke and asking for money.
In "Gatekeeper", when Scott Delfino, a womanizer, leaves after sleeping with Ashley Foulandi, Tanner stalks and ambushes him, killing him like he murdered the other two victims. Placing mementos in a scrapbook, he overhears Jake Preston talking with another of his tenants, named Colin Kirkland, whom he tries to offer cigarettes and then gives him a video game he isn't supposed to have. Tanner tracks down Jake at his home and murders him in the middle of his game as Colin watches from Jake's cell phone. He then goes to work, but is surprised by Ashley when she sees his scrapbook, and they get into a struggle when Ashley tries to escape. Tanner accidentally sends her over a stair railing, sending her falling three stories down to her death. Panicking, Tanner garrotes her post-mortem and dumps her in a trash bin elsewhere.
Tanner then breaks into Hannah's house, seeing that she is going into labor, and kills her brother. Completely delusional at this point, Tanner shows Hannah his new scrapbook as proof that he can protect their child. However, Hannah keeps telling Tanner that he is weak. Hannah begins going into labor, and Tanner insists he can deliver the baby himself. Reid and Morgan burst into the room, and a panicking Tanner threatens Hannah with scissors. Reid convinces him to hand over the weapon by claiming that the baby's umbilical cord is wrapped around his neck, and Morgan grabs Tanner and drags him away as Reid assists Hannah in childbirth, with Hannah naming her newborn son after Reid. Due to his current mental nature, it is very likely that Tanner is placed in an institution afterwards.
Tanner targeted males of various ages and races who he observed and perceived to be bad influences after observing them from a window in his room interacting with the tenants of the apartment building he worked in. He would stalk these people, blitz-attack them at night, and strangle them to death from behind with some form of household wire (as a reference to his son's death), each ligature being stronger than the last. His signature was taking photographs of them before and after death, taking a souvenir (such as a wallet or a piece of hair), and placing them, along with the murder weapon, in a scrapbook. The first three victims were killed in the downtown areas of Boston, but Jake Preston was killed inside his own home. When he accidentally killed Ashley Fouladi, he threw her down a three-story stairwell, strangled her post-mortem after panicking, dumped her body in another location, and covered it with a blanket out of remorse. The victims were initially killed within the span of a month, but as Hannah grew closer to giving birth to her new child, Tanner began killing more frequently.
The unsub is a dangerous male voyeur who feeds off from the thrill of knowing that his victims don't see him. Typically, voyeurs are nonviolent and content enough to remain as bystanders, but this particular voyeur is on a mission that includes taking action. In that mission, he did not hesitate to kill a teenage boy, so anyone who stands between him and his goal is at risk. He is documenting his murders by taking pictures and collecting personal connections to his victims (their wallets and their hair), which means it is important to him to have proof of what he is doing. Voyeurs like to create fantasies in their heads of what the objects of their obsessions should be like. This unsub creates a character identity instead of a reality, and when a person steps outside the parameters that the unsub has set for them, he strikes. He seems to be triggered by males acting on what he perceives to be character flaws, usually perceived by a majority as seemingly innocuous infractions, but in the unsub's extreme psychosis, the significance of these flaws are magnified.
Based on the planning and the sophistication of the murders, he is believed to be aged in his late 30s or 40s, and the geographic profile reveals that he isn't only dumping his victims' bodies downtown, he is hunting there as well. He is essentially a fly on the wall, being able to learn secrets without anyone realizing he is there. As a result, he will have a job that involves very little social interaction, but puts him in a position where he can observe other people unnoticed. He may work as a service worker, groundskeeper, or any other job that requires being invisible. In addition, the unsub is approaching some sort of perceived deadline, and the closer it gets, the more erratic and dangerous he will become.
- Michael Dunsmore (a drug addict; strangled with fishing line)
- Ryan Maxford (a beggar; strangled with picture wire)
- November 3: Scott "Scotty" Delfino (had sex with multiple women; strangled with cable wire)
- November 4: Jake Preston (gave his friend cigarettes and video games; killed in his home; strangled with unspecified wire)
- November 5: Ashley Fouladi (a tenant who slept with Scott Delfino; stalked for at least two days; later accidentally killed after throwing her down a stairwell, then strangled post-mortem with an unspecified ligature)
- November 5-6: Matt (Hannah's brother; strangled with an unspecified ligature)
- November 6: The standoff at Matt's house:
- Hannah Johnson (his ex-wife; held captive and threatened with a pair of scissors; was rescued)
- Spencer Johnson (Hannah's then-unborn son; indirectly attempted; nearly died as a result of Tanner taking Hannah hostage)
- Unspecified year-2013:
- Numerous unnamed tenants
- At least two unnamed tenants (connected to Michael Dunsmore and Ryan Maxford, respectively)
- Colin Kirkland (Jake Preston's friend)
- Tanner is similar to Season One criminal Walter Kern, a.k.a. "The Keystone Killer". Both were organized serial killers who devolved, had an M.O. that involved strangulation (although Tanner used ligatures, while Walter used manual strangulation), stalked and took pictures of their victims, took their licenses and put them in scrapbooks, and killed one of their victims differently from the rest (Tanner killed his fifth victim by throwing her down a stairwell; Walter killed his last victim by suffocating her with a plastic bag).
- Tanner is also similar to Donnie Bidwell, a serial killer who appeared in the previous season. Both were motivated by their wives divorcing them, and were triggered to commit their killings after learning that their ex-wives were pregnant with another man's baby. They also stalked their victims and removed body parts post-mortem.