The MurdersThis was an important event in the series because one of the victims (Laura Moser) was murdered in front of her two young sons - Dexter and Brian Moser. The two boys witnessed their mother and three others being butchered with a chainsaw, and then they were left behind to sit in the gore for two days.
Officer Harry Morgan was called to the scene, and he rescued (and adopted) Dexter, thus separating the brothers. Over time, Harry began to notice something off about Dexter. He eventually found a grave of animal bones, and Dexter admitted that he had killed the animals.
As a homicide detective, Harry realized that Dexter's dark urges would likely cause him to victimize humans later on in life. To help Dexter avoid prison, Harry taught him a code to live by that would protect him. Under the code, Dexter was allowed to murder deserving individuals without falling under suspicion by law enforcement.