There's nothing good or moral about a serial killer. What Dexter does is murder, pure and simple.
And he is already responsible for the death of a lot of innocent people, which didn't stop him.
I don't think ANY of you would applaud a serial killer psycho vigilante to run around your neighborhood, chopping people into pieces.
So I never saw any appeal in that show.
Oh I dunno, I suppose if I had a family member killed by someone and they got off scot free on a dang technicality, I might definitely support that.