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
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.