When Dexter Morgan was three-years-old, he witnessed his mother being chain-sawed to death inside a shipping container, referred to as the Shipping Yard Massacre. Two days later, he was found sitting in blood and rescued by a homicide detective, Harry Morgan.

From an early age, Dexter showed signs of psychopathic tendencies (killing and dismembering animals). His adoptive father, Harry, (a homicide detective) realized that Dexter was a potential serial killer.

After attempts to curb Dexter's behavior failed, Harry concluded that Dexter's need to kill must be focused. Making the best out of a bad situation, Harry trained Dexter as a vigilante killer (with guidance from Dr. Evelyn Vogel) to solely hunt other murderers (arguably making society safer). To avoid arrest, Dexter learned to follow a set of guidelines, referred to as the Code of Harry.

After Harry died, Dexter bought a fishing boat and began to bury his victims at sea. For easier disposal, Dexter generally dismembered his victims, packed the body parts into several trash bags, and dropped them off his boat into deep water. During Season Two, scuba divers discovered some of the body bags, and they were labeled the Bay Harbor Butcher 18.

Because The Bay Harbor Butcher seemingly only killed other killers, much of the public expressed support for the unknown figure (including Rita's own mother, who believed that he should be left alone). This led to the creation of The Dark Defender, a comic book hero concept based on The Bay Harbor Butcher.

By the end of the OG and New Blood series, Dexter had killed at least 151 people.