This season is supposed to be a bunch of mini-villains instead of one big bad.
I think they're finally ready for Dexter himself to be the big bad of the season.
One can only hope that he was able to dispose of the crime scene properly after his post-kill rally. He had the boat nearby, so maybe he was able to dispose of the body unseen. I think this is not at all certain based on his state of mind at the time however.
It just seemed so out of the blue that I'm sure there's some bigger agenda that the writers have in mind.
I'm looking forward to seeing how the writers are going to deal with Astor in the story. I wouldn't be surprised that of all the unkillable characters who might discover Dexter's secret, she would be the first. Something is definitely afoot in how Astor and Dexter's relationship is going to be developed.