Throughout season one of Mindhunter, brief scenes of a man seemingly in the midst of planning a murder are sprinkled throughout. He's never named outright, and we don't see him actually kill anyone. So, what's his deal? His appearances might seem random at first, but the fact that he works at ADT security should tip you off — he's actually Dennis Rader, better known as the BTK Killer. Rader (whose nickname comes from the way he would "bind, torture, and kill" his victims) murdered 10 people in Kansas between 1974 and 1991.
Mindhunter's first season takes place in 1977, which means the BTK Killer had slain five people by that point. The show follows him into season two as he stalks potential victims via his job as a security system installer, which gave him unfettered access to peoples' homes. Following his crimes, he sent taunting letters to the police and news outlets bragging about what he'd done, and was finally apprehended in 2005.
Sonny Valicenti, who played FNG in the TV series Platonic and Dean in Office Christmas Party, is the actor behind the mustachioed serial killer on the show. He's only billed as "ADT Serviceman" on IMDB, but the second season digs more into his arc.