Matthews and Harry bring in the British chick years ago to evaluate Dexter as a small boy. She knows from the get go Dexter is doomed. That's when they try and make a wrong right by creating the perfect killing machine to clean the streets of all the killers that fall through the cracks. Harry's job was to adopt Dexter as he had no children at the time and he felt guilty over Dexter's mother's death. From there, he was to instill "Harry's Code" once Dexter's psychopathic tendencies began to clearly show.
When Dexter finally kills someone, Harry has second thoughts and confronts Matthews. Matthews kills Harry because Harry can kill his quickly moving career and she leaves to go back to England.
And Matthews has been "protecting" Dexter ever since...
Great theory. I think the fact that Matthews has always seemed to deflect or protect Dexter in a very nuanced way gives that theory legs.
I almost wonder if Dexter was "Jason Bourned (or given the Dark Passenger)" into what he has become. He is usually very reserved and protective of who he is with everyone, but his immediate hostile and emotional reactions to Vogel shows there might be familiarity there.
Has she been brought back to right the ship as Dexter veers off course more and more?
Is the new serial killed somehow tied to her and was a past experimental subject before or after Dexter? Was this person deprogrammed and retired, and has become active again by being brought back into action by Vogel and Matthews?
The fact that Matthews has come out of early retirement (did anyone pick up how that happened?) makes me feel that their experiment has gone awry and they need to remedy it before Dex is caught from his changing emotional responses and internal crisis of conscience, psychoanalyzed by a third party, and the cat is let out of the bag about them.
Just my additional two cents...