I'm assuming Dexter had to fake his own death to get away with dumping his sister in the ocean and killing Oliver Saxon/Daniel Vogel. Let's face it.. the tapes clearly show he intended to kill Saxon.. and there must be security monitoring the hospital.. He knew that, so he had to fake his death to avoid prison.
I watched the last scene of Season 4 with Trinity and a lot more made sense.. but it still sucked gorilla ****.
These writers have to stop making these "Choose your own interpretation" endings.. They really blow.