Dexter executes criminals who deserve to die. The show didn't worry excessively about Jimmy Smits' brother. It didn't worry at all about the guy in the RR because he was incredibly obnoxious when Dexter was grieving. Kicking Our Hero when he was down? How very satisfying to see the worthless fuck get his!
Rita was killed off as a process of turning the show into nothing but vicarious murder of people the viewer is presumed not to like. That's the show's premise, therefore he is always able to find his prey, kill his prey and escape capture. Since that's all that's going on, it doesn't matter how ludicrous the script is.
It was something else in the beginning, a comedy about a crazy man who was going sane. There was the comedy of a man so clueless he thought he was a monster nevertheless heroically committing to being the best brother and lover he could be. Angel was the moral center of the show. LaGuerta was the center of a sharply observed workplace drama about another pursuit of criminals and "justice" that, ironically, had no code, not even Harry's deranged imposition on Dexter.
But in one sense criticizing what Dexter does now
is foolish. What he does is about a meaningful as a Superman catching bank robbers.