Detailed Explanation
The Season 1 killer was Joe Miller—Ellie's own husband, the father of Danny's best friend, and a man the entire community trusted. Joe had developed an obsessive fixation on Danny and killed him when the boy threatened to tell people about Joe's inappropriate behavior. The reveal was devastating because it came from within Ellie's own home, destroying her life completely. Actress Olivia Colman wasn't told until filming the reveal scene, making her reaction painfully authentic.