Dexter would be an indefensible character in the real world, but thankfully it's all fiction.
I could NOT get behind a "person" like Dexter if such a person actually existed. I oppose capital punishment for any reason, because people are imperfect and can make mistakes and innocent people die.
I see no harm in indulging in the escapist fantasy of a TV show, however. In his world, the character makes perfect sense.
Not sure what my point here was except that trying to apply real world logic to the Dexter character seems kind of absurd. He's about as realistic a character as Batman or Superman. May as well ask if The Hulk would be liable for property damage.