The episode where Dexter kills his brother Brian is “Born Free.” It is the season-one finale of Dexter, in which Rudy Cooper is revealed to be Brian Moser, Dexter’s long-lost biological brother and the Ice Truck Killer.
Brian tries to force Dexter into accepting him as family by arranging Debra’s murder. Dexter refuses to let Brian kill her and later captures him after Brian attempts another attack. Dexter then kills Brian by cutting his throat and stages the death as a suicide, protecting Debra while concealing his own involvement.
“Born Free” aired on December 17, 2006, and is the twelfth episode of the first season. “Seeing Red” is a different season-one episode, while “Popping Cherry” and “The Return” occur earlier in the season and do not depict Brian’s death.