Comeuppance (S01E03)

Following the murder of four children, Ian Brady's arrogance grows, prompting him to seek new followers for his twisted ideology. He and Myra Hindley entice a 17-year-old boy to their home, where Brady plans to murder him. They also draw in David Smith, who is manipulated into helping with the gruesome aftermath. However, instead of becoming part of their sinister gang, David bravely contacts the authorities to report the horrific crime he has just seen unfold.