Theo was born to a serial killer couple who were also members of a cult called The Believers. At some point, they kidnapped former FBI agent Owen Quinn, and tortured him after he realized that the cult was responsible for several murders. While Quinn was captive, Theo and Quinn grew fond of each other. After turning 18, he enrolled in Reid's criminology classes, and got a girlfriend, April Pearson. After a few months, Theo's parents kidnapped his girlfriend and told him that, if he did not kill Quinn, they would kill her. Instead, he shot and killed both of his parents, anonymously told Reid where to find Quinn, and ran away with April.
Knowing that the police and the cult were going to find him, he decided to hide at his girlfriend's house. Before he could get there, he was found by two members of The Believers. He shot and killed both of them and escaped to April's house.
When the BAU arrived at April's house, the couple explained to the FBI that the leader of the cult, Benjamin David Merva, was in fact a serial killer and could be found in a barn outside of D.C. and Baltimore.
Reid and Theo conspired to escape their captors by causing a distraction and allowing Garcia time to escape. During the escape and subsequent evacuation, however, Theo was shot and killed, as he was not loyal to the cult.