Here's the first principle of Dexter: killing people is wrong. But no, you protest. It's a show about a serial killer. The protagonist kills people. Clearly the first principle of Dexter is that killing people is ok sometimes, under certain circumstances - the very opposite of what I suggested.
To which I say: hold on there, bronco. Think about what you just said. It's a show about a serial killer. Not a show about a superhero who seeks out bad guys and delivers justice in a viscerally satisfying way (i.e., by killing them). Not a show about a secret agent, cop, or other sanctioned operative whose conscience has been bought or trained out of existence, giving them a pass. Those are scenarios in which killing is ok, sometimes, under certain circumstances. When the means justify the ends. Those scenarios tend to be all about the 'ends': peace, happiness, building a better tomorrow, forestalling terrible disaster, saving innocents. Et cetera.
Dexter does not offer us those free passes.
On your average crime show, if the protagonist killed a killer, that would be a happy ending. There might be a note of pathos at the end, but we've all become very familiar with a narrative in which killing the bad guy is a cue for the credits, for a sappy kiss, for a walk off into the sunset. On Dexter, when our protagonist kills a killer it's a sign of his continuing mental illness.
The whole show is a great big middle finger pointing at conventional narratives that excuse the characters and the audience from exercising their consciences. We may root for Dexter, but we are reminded at every step along the way that he's a monster and what he does is wrong.
That's brilliant. Even more brilliant is that while that middle finger is waggling, the show draws us in by feeding us the exact same viscerally satisfying narrative that we hunger for. Find the bad guys! Kill them! And Dexter does. It's the televisual equivalent of a hedge fund.