What are the moral issues involved in the TV drama Dexter?

The central moral quandary involved with Showtime's series Dexter, which ran from 2006 to 2013, involves the age-old question of when do ends justify means. Dexter's "protagonist," of course, is a serial killer. There is little or nothing that historically can be said in defense of serial killers. The moral quandary involved in Dexter, however, lies in the titular character's practice of limiting his murders to those of other serial killers. Dexter works with the police to solve crimes and is instrumental in the resolution of some of the toughest cases. His role of forensic analyst and his passion for murder make him both invaluable for the identification of killers and for the execution of justice in cases where the formal and legal judicial process may fail.
The moral dilemma at the center of Dexter, as noted, can be summarized as the "ends justify the means" argument in which virtually any level of depravity can be excused in the service of a larger cause. Note, for instance, the following quote by the character Dexter:

"How many more bodies would there have been had I not gotten to those killers? I didn't want to save lives, but save lives I did."

This semi-rhetorical question by the show's protagonist is the central theme of Dexter. The unpalatable notion that extrajudicial executions may serve a larger public good is a part of everyday life around the world, for example, in the Philippines today. It is a question that will never be definitively resolved, as there will always be a thirst for vengeance that can only be executed outside the boundaries of the law.