Part of the draw of Mindhunter is figuring out how the characters stack up to the real detectives and serial killers in the show, but in Season 2 that starts to take a disturbing turn. As Holden and Bill investigate the Atlanta Child Murders, the BTK Killer, and meet with the likes of Charles Manson, it comes to light that Bill's son Brian Tench was part of a group of boys who killed a neighbor boy. Yes, really.
Early in the season, the body of a toddler is found in a house that Nancy, Tench's wife, is the realtor for. She's pretty shaken up about it, as anyone would be, and sends her FBI detective husband to get her name off of the "for sale" sign in the yard, lest the murderer comes looking for her due to her connection of the house. When he does, he befriends the local cop and gets a glimpse of the scene — the little boy was found suffocated in the basement and put on a makeshift cross. Sounds professional, like the kind of guy Bill and Holden hunt down all day. Things take a dark turn when it's revealed that the boy was left for dead by Brian, Bill's adopted son, and a group of friends. While Brian apparently didn't KILL the kid, it was his idea to put him on the cross and he took part it in.
Throughout the season, viewers watch as Brian regresses, stops talking, starts wetting the bed, and is subject to counselor appointments. Nancy is hopeful that he can ignore or get past the trauma. Bill, knowing what he knows about murderers, isn't so sure. The juxtaposition of him considering what makes a killer — and the fact that other young boys in Atlanta are being murdered en masse — makes it especially poignant. Given that the detectives are based on real FBI agents — it's not wild to wonder if Brian is based on someone real or worse, the son of an FBI detective who grew up troubled and potentially violent.
Tench is based on Robert Ressler, an agent who worked closely with John E. Douglas (who inspired the character of Holden) and reportedly invented the term "serial killer". The real Ressler did have a son, but he doesn't share any characteristics with Brian at all. So, whew.
But the character does make things a little more interesting, and disturbing. Tench seems pretty convinced that his adopted son is a psychopath — he takes him out for ice cream one day and tells a story about a fish. His son's only question, the first words he's spoken in weeks is: "Did the fish die?" Not a good sign, right? He confides in Wendy but doesn't tell Holden for a while, and it forces him to work a little less and handle things at home, which are not going well. You can see him slowly breaking episode by episode, even as he throws everything he has into hunting down the Atlanta Child Killer.
So rest assured that Ressler didn't have to deal with any of this in real life, but be prepared for some wild plot twists if Mindhunter is picked up for Season 3 and they keep on with Brian's story.