The killer is Cassie's boyfriend, and I think that he is Dr Vogel's son. Remember at dinner Hannah asked her how she got into it this whole thing, and Vogel said that it was a story from a long time ago that she doesnt want to get into. If the TBS is indeed her son and she helped create him, that would explain why he keeps sending all of his trophies to her. Plus, he has an accent similar to the one that Vogel has.
I think that Quinn planted the blood on Cassie to try and frame Zach, and he now knows that Dexter is withholding information on him. Everything is closing in on Dexter for these final 4 episodes, including...
1. Quinn is onto him for covering up for Zach (even though Zach didn't kill Cassie).
2. The Brain Surgeon is stalking Dexter (he left Zach's dead body in Dexter's apartment, for one) like he stalks his prey, and he is jealous of Dexter's relationship with Vogel.
3. Even though she let Hannah go, she is still fuming that Dexter wants to be with her. Couple that with the fact that Deb and Quinn get close again in the previews, who knows what happens. Maybe Deb falls in love with Quinn again (or maybe she still is) and he is able to convince her to go after her brother once and for all.
I guess that the TBS kills Vogel, and Dexter kills him because he looks at Vogel like a mother and TBS is another long lost brother (so to speak) that he needed to dispose of (like the Ice Truck Killer). Dexter then has to choose between Deb and Hannah once and for all, and he kills Deb and Quinn to allow for his escape and he runs away with Hannah and Harrison (remember, he has a passport guy) forever.
This will probably change next week, but it's what I think as of now.