Thing you got to remember about Dexter is that he doesn't randomly kill just anybody, he kills bad pople like killers who get away.
I've seen it thru S2, and to that point, there were no bad episodes or even mediocre ones. Only good, great, and mind-blowing!
I don't buy DVD's. I refuse. I rent or borrow. So it says a lot that I own both seasons of Dexter. ~String
Dexter is a work in progress, like all serialized dramas. The track record for them is extraordinarily poor. It is necessary for the dramatic resolution of the story to be artificially delayed. Thus, Dexter cannot go finally, completely sane til the end of the show's run, even if it means Spoiler: Dexter's choice that the ultimately logical consequence of rejecting Rudy's acceptance of him as he is must be delayed by creating a lover who, remarkably, will accept him as he is but who---just like his brother require that he reject his decent life, even to the conveniently melodramatic extreme of making it a choice between someone's life or death. And yet again, the choice of his sane life means doing his same old work! What an amazing coincidence! As can be seen, Dexter, like so many character driven shows (aka soaps,) fallen to telling the same story again. But not all hope is lost---Dexter has found out not just that Harry lied to him but regretted what he did to Dexter. He hasn't had time for that and what it means to really sink in yet. Understanding that Harry was nuts is an inescapable part of his recovery. The thing is, if he doesn't realize it sooner or later, the show will just be whether a charming serial killer can have it all---sister, girl friend, meaningful work and a fun hobby. Having the show turn out to be the escapades of Dexter the same at the end as at the beginning would be a complete failure artistically. I suppose part of their problem is that a man going sane seems like a happy ending, a comedy. (Dexter has lots of humor.) But both the plausible and morally conformist endings would be pathetic or tragic.
Dexter deserves a happy ending. The only happy ending is one where he stays true to himself - as insane as he's ever been - but with at least some people in his life seeing him for who he truly is and accepting him. At very minimum, I'd say that's Deb and Rita (and the kids, but kids accept what they're told is okay). That's enough for me. Dexter will never be sane. It's implausible and it would violate the integrity of the character. The point of the story is that for him, insanity is sanity.
Does he ever make a mistake? Meaning, has he ever killed somebody he thinks is a criminal but is actually innocent? I think I would. See above.
Spoiler: Spoiler about Season 3 He does kill someone in self-defense at one point, when the person he tries to kill is fighting someone else. That person turns on him with a knife, while Dex's intended victim flees. Dexter has to kill the knife-wielding guy, in self-defense. And yes, he feels bad about it. Whether that guy is really "innocent" or not, though---well, I can't say at this point.
Dexter getting away with murder over and over is absurd. One of the few things the character voiceovers can be trusted on is that he will be caught or killed. It is hard enough to accept that Dexter is the greatest detective in the universe, but that is a premise. Dexter's permanent escape is a consequence. An absurd premise may keep you from getting into a show as it is. But an absurd consequence falsifies the show artistically. If Dexter had been canceled at the end of the second season, Dexter, it is implied, would have Spoiler: Dexter's choice continued with Rita and Deb in his life and his hobby. But the idea that Dexter has truly come to terms with Harry's suicide is nonsense. The necessity for an open ending to a serialized show means playing it both ways. Dexter has already had Harry, Brian and Lila eager to accept him as he is. As I said, the plot has arbitrarily ordered Brian and Lila to make Dexter execute them as 1.) killers who 2.) try to kill his loved ones. And Harry of course was the one who started him off on it all anyhow. Dexter has already chosen normal feelings (sanity) over insanity (his hobby) repeatedly. How is it possible to claim that Dexter has not made any significant choices, that the point of the show is merely how Dexter gets away with it all? It's plotted pretty well but Dexter's escapes already defy a close look! Given the character letting the kid off in season 1 or his inability to kill the blind man in season 2 or his inability to kill Paul or Doakes, the idea that Dexter feels only pleasure in his avoication has already been contradicted. His character is explicitly shown to be on one level unhappy with his isolation from humanity and feeling guilt. The man had seriously decided to confess! Only a weird plot ended up reversing that. It may not seem satisfying if the real appeal of Dexter is the vigilantism, but quitting and getting away with the past murders would be a happy ending. (My guess is that there have been serial murderers who have slowly come to their senses.) Comedy is about conflict. Generally, the conflicts are allowed to play out without consequence (the hallmark of farce) or papered over with sentimental nonsense in the last fifteen minutes. But a comedy that ends honestly has a special name---tragicomedy. Dexter the show could be one of this rare breed.