In the realm of superhero comics (and movies), there's been (since Watchmen at least) a realization that what superheros allegedly do, i.e. beat people up, requires a certain psychosis, and comics like The Punisher make that explicit.
With the "Dexter" books by Jeff Lindsay and the TV show based on them, this is approached from the other side, where the main character is beset (thanks to childhood trauma) with the need to kill, that his foster-father channels into killing according to a code, i.e. only killing bad guys. To some degree, the comparison is made ironically: Dexter knows he's sick and that the world would be better if he were dead, but clearly we're supposed to root for him, both because the show's villains are obviously worse and because of Dexter's intrinsic likability.
The show is trying to pick at the moral sensibilities relevant to our enjoyment of any kind of splatter-fest film fare, for one. Some of us, for whatever reason (there's certainly been lots of speculation and study on this, but I'm going to assume that we as audience members don't already have a clear philosophical theory about it), enjoy violence-filled entertainment, and two elements enjoyment in this are 1) when something over-the-top gross happens (and I'll argue that it doesn't even have to be realistic, not that we normal people would necessarily know what constitutes realistic in this area, but it includes, e.g. the obviously fake black knight "only a flesh wound" spurting in Monty Python and the Holy Grail), and 2) when particularly bad guys get put down, preferably with poetic justice.
So whereas #1 just relates to something like catharsis re. violence, #2 plays on our sense of justice, and it works even when justice is perverted to some degree, as when a bad guy (Dexter) finally takes out a worse guy. I'd argue that the worse guy needn't even be worse, as with stories with true anti-heroes where you're supposed to identify with the bad guy and (sort of) rejoice as he triumphs over wussy good (I know I've witnessed such stories but none are coming to mind at the moment); I don't want to talk about that kind of story now, but it's interesting as a limit case for the current topic.
Now, Dexter only works as a hero because we know he's fictional, but there's relevance to Spinoza's (or more precisely pseudo-Freudian) views on ethics too. He is depicted as determined by his trauma; he can't just choose to not kill. He can only choose, and sometimes with difficulty, the circumstances by which he kills, and he uses explicitly moral reasoning (both utilitarian and retributive) to make these decisions.
He also gives us a picture of why purely rule-based ethical systems don't work. He has (nominally; the stories are inconsistent in this respect) no natural impulses to do the right thing, and so has to keep his "code" clearly in mind so as not to be totally immoral, and this inevitably results in miscalculations that lead to bad consequences. Now, the show actually depicts these lapses as his failure to follow his code consistently, but this gets a little complicated, as parts of the code are not directly moral but merely matters of caution, and it's his incompetence to deal with the subtle difference between them that runs him into trouble. In short, if you have a rule-based morality, you also have to have the judgment to apply the rules appropriately, and this judgment in turn can't solely come from the system of rules.
So, we've got sublimation of our aggressiveness and fear of death going on in our enjoyment of this, but our joy in seeing the badder guy get it also feeds the aggression inherent in our self-righteousness, our sense of justice as revenge (which doesn't have to be revenge for US, but for some wrong depicted in some way on screen or in the text with enough detail for us to feel outraged... and it doesn't take much detail to do this).
Incidentally, contra Demasio, despite Dexter's stated inability to feel emotions (that being the whole thing that allows him to be a killer), he has no trouble being decisive, which means that something else is going on here, either blockage of certain emotions and not others (which is also stated in the story at times) or merely a psychologically illiterate portrayal of the character (which I'm also willing to admit).
What about Dexter qua super hero? He has a secret identity and all the risks involved in it being found out and thus having to protect his family and friends from the bad guys and juggle his super and regular life. What are his powers? Well, daring. The inability to feel (Much like Daredevil, the man with no fear!). Like your modern angsty superhero and/or mutant, his gift here is also a curse, setting him apart from others. His "dark passenger" also makes him somewhat physically stronger than one would expect, and his training in espionage to support his hobby gives him a jump on the bad guys. I'm not sure I can dredge much philosophical insight out of this, but we'll return to it with a review in a bit of Kick-ass, a film and comic that explores the regular person-superhero connection. Suffice it to say at this point that the trope has changed from mere wish fulfillment (a la Superman) to something that is more complex, or at least fulfills more complex, more often repressed wishes.