In Dexter,the titular serial killer operates under a strict code given by his adoptive father Harry Morgan, with these rules guiding every aspect of Dexter Morgan’s life. Dexter Morgan developed a homicidal Dark enger after being found in a pool of his murdered mother’s blood as a toddler, with his police officer father Harry Morgan eventually teaching him a set of rules that use his overbearing urge to kill for good. While Dexter’s guidelines help to avoid him being caught, they also give him a moral code used to justify his murders.

Dexter season 8 reveals that the Code of Harry was developed by Harry Morgan and Dr. Evelyn Vogel, who realized that Dexter exhibited sociopathic tendencies and an inclination to murder as a child. In order to save his son from being locked up in an institution or receiving the death sentence, Harry decided that he would help use Dexter’s Dark enger to rid the world of serial killers or murderers who slipped through the cracks of the justice system. However, these rules that Harry created still weighed on his conscience, with the police officer eventually taking his own life after seeing Dexter in action.

Rule One: Don't Get Caught

Harry Teaches Dexter How To Evade Suspicion

First and foremost, the most important rule in Dexter’s Code of Harry is “don’t get caught.” If Dexter were to be caught, he would face imprisonment or execution, with Harry even taking a young Dexter to see an execution first-hand as a cautionary measure. The majority of the other rules in the Code of Harry are used to ensure that the first rule is followed.

Dexter often reiterates that the first and second rules of Harry Morgan's code are the most important.

Dexter often reiterates that the first and second rules of Harry Morgan's code are the most important. There are several more specific rules, such as "leave no traces or evidence", "Fake emotions and normality", and "never make a scene" that are all steps to ensure the first rule of Harry Morgan's code is followed. It's also this rule that shows how much Harry cares about his son. Of all the rules and advice he left for Dexter, the majority were there to ensure Dexter's Dark enger didn't lead him to prison (or, worse, a death sentence).

The first rule of Harry Morgan's code includes the importance of blending in socially and maintaining a “normal” appearance, faking his emotions, and leaving no evidence or traces of his crimes (such as with Dexter's M99/Ketamine drug). Leaving no evidence also means never making a scene by getting emotional or having clouded judgment - the first rule in Harry Morgan's code ensures Dexter can continue killing so long as the future Bay Harbor Butcher mains a methodical, clinical approach.

Rule Two: Never Kill An Innocent

How Harry Keeps The World Safe From Dexter

If the rule of "don't get caught" is how Harry Morgan kept Dexter safe from the law, "don't kill innocents" is how he kept the world safe from Dexter. Once he discovered his son's dark impulses and murderous urges, Harry was clear that he wouldn't help Dexter become a traditional serial killer. Instead, he would help his son use his Dark enger as a force for good. This is only possible if Dexter is incredibly selective with his victims and - most importantly - never kills innocent people.

The rest of the rules of Harry Morgan's code that aren't related to the first rule are those that keep Dexter’s Dark enger controlled and operating under a moral guideline. The Code of Harry is strict in the manner in which Dexter can kill and who he can take as his victims. After never getting caught, the most important rule in Dexter’s Code of Harry is to never kill an innocent.

However, this rule can sometimes be broken in order to adhere to rule number one, as Dexter's main character must avoid being discovered at all times. This means there were times when Dexter had to take the life of an innocent to avoid being exposed - and from his teachings and flashback scenes, it's likely Harry knew this would be the case.

Rule Three: Targets Must Be Killers Who Evaded The Justice System

Harry Was Specific About The Criminals Dexter Killed

Training Dexter on being a killer who killed other killers eventually psychologically broke Harry, a lifelong police officer. However, while the guilt of creating such a violent presence capable of operating outside the law eventually led to Dexter's father taking his own life, the Code of Harry Morgan is still clear about the legal system. Harry was clear to Dexter - he cannot choose victims who have yet to be caught by the proper authorities.

This rule is the next step of Harry Morgan's code after not killing innocents. It's not enough that Dexter targets people he suspects to be killers or criminals - they must have been charged first, and viewed as dangerous by law enforcement. Harry Morgan's code helps Dexter correct the mistakes of the legal system, not act as a moral judge. If Dexter is searching for a victim, he also must be sure that the person he kills is a murderer, which involves intense research by Michael C. Hall’s character. In conjunction with this important rule, the Code of Harry states that Dexter’s victims must be killers who have evaded the justice system or slipped through its cracks.

Rule Four: Killing Must Serve A Purpose

The Spiritual Core Of Harry Morgan's Code

Of all of Harry's rules in Dexter, it's "killing must serve a purpose" which is the most important in many ways. It's this rule that captures the reason why Harry decides to help Dexter harness his Dark enger, beyond simply being a protective parent. As a police officer, Harry is against murder. However, he also knows that the legal system allows many killers to go free. It's this knowledge that led to Harry creating the code - it's a set of guidelines that harness Dexter's impulses for a purpose and gives them reason.

So important was this to Harry Morgan that he ensured specifically stating it was part of the code. Dexter must only kill when there's a clear, solid reason. Knowing that Dexter’s urge to kill has no inherent motivation for his bloodthirst, Harry Morgan created the rule that all of his kills must have a purpose, otherwise, it’s “just plain murder.” Dexter’s kills must have a purpose in preventing the deaths of innocents or “balancing the books,” as his murders cannot be motivated strictly by vengeance.

Rule Five: No Preemptive Killing

Dexter Must Not Make The First Move

Of all the rules Harry Morgan left for Dexter (that aren't methods for keeping his identity concealed), "no preemptive killing" is one of the most specific. Harry knew that Dexter was smart enough to spot other serial killers before he'd gained sufficient evidence, and that in many cases he'd be correct. This is why Harry ensured Dexter would never make a mistake by including the rule preventing any preemptive killing.

It's not enough for Dexter to rely on his understanding of serial killer psychology to identify a potential target - he must have proof. Moreover, Dexter must also never act to prevent crimes before they happen. In the Dexter episode "Seeing Red", the 10th episode of season 10, Dexter mentions that Harry was very clear about this particular rule, and was also candid that sometimes he finds breaking it tempting. With the prequel series Dexter: Original Sin arriving in December, it's likely that viewers will get a much more detailed version of Harry Morgan's code and the origins behind each rule.