The Reid Decision That Has Criminal Minds Fans Rewatching This Episode

In the beginning of the episode, teenager Nathan Harris (played by the late Anton Yelchin) accosts Dr. Spencer Reid on the subway, saying he saw him at a Georgetown University lecture on sexual sadism and "thought [he] seemed cool." Nathan tells Reid he's interested in true crime and the nature versus nurture debate when it comes to criminal proclivities, and then asks about the motives of serial killers specifically in the deaths of sex workers. At that point, Reid clearly thinks the conversation seems off, and Nathan reveals some details of a murder that Reid says he's never seen before. Reid asks the boy if he wants to come to the BAU with him, scaring him off. 

The conversation leads Reid and the team to discover that there's a serial killer at work stabbing and cutting off the hair of sex workers, and step one of the investigation is to locate this teenager who knows so much. Nathan hangs around the sex workers and admits during questioning to thinking about killing them — but his willingness to come in shows he's also looking for help. He's still with the BAU when another murder happens quickly afterward, so it's clear he isn't the killer they're looking for. However, a peek into Nathan's inner psyche and a formal psych evaluation shows he's struggling with some dark thoughts — and that it's more a matter of when he'll become a killer than if he will. Then, a third murder takes place. 

Later in the episode, Reid saves Nathan's life, but what the evaluation has revealed about the teen has him wrestling with that decision. Nathan is innocent, at least right now, but he's likely a future serial killer. "He wanted me to let me to let him die," Reid says, then wonders, "How many lives did I risk in the future?"