SPOILERS! DO NOT READ IF YOU HAVEN'T SEEN THE FINALE.
Trinity did was not Rita's killer. It is much worse than that.
Dexter has finally lost control of his Dark Passenger by drifting from "The Code".
Yes, the sad truth is that Dexter killed Rita.
In his mind, Dexter created the scene in which he sent Rita away in a taxi to protect her.
>>> Search your feelings. You know this to be true.
This season has been about balancing the relationship between a man, his killer instinct and his family.
Dexter wades into the mind of a similar killer who seems to have successfully created a balance.
Emotion, empathy, and errors creep into Dexter's life and "The Code" is being set aside.
Dexter kills an innocent man and the Dark Passenger begins to work on his own.
Dexter wants to protect his family from what Trinity is doing to his family, but he doesn't know how.
The Dark Passenger knows that Rita cannot be harmed in any other way, if she is dead.
This cleans up the time-line problem everyone has been discussing and how Trinity found Rita.
If you watch the interview between Hall and Lithgow, you will see that it is Lithgow that says
that Trinity killed Rita and Hall just smugly agrees. Please remember how secretly plot points
And finally, please don't kill the messenger. I'm sure Dexter will find me sooner or lat.............