Dr. Vogel shows Dexter her recordings of Harry describing his step-son's violent traits (by the way, it seems unlikely that Harry gave her permission to videotape those conversations). Long story short, it turns out that Vogel was the one who gave Harry this idea of channeling Dex's urges and helped him create the Code. She now needs Dexter's help, as she believes the murder from last week was committed by one of her patients, since she's received that victim's missing part of the brain.
The new body appears almost instantly, Dexter manages to identify the suspect by a fingerprint on the murder weapon, and although Vogel seems skeptical about the candidate, Dex chooses to ignore her.
Debra, in a meanwhile, is on a treasure hunt. She figured out where Briggs kept the jewelry, and went there, followed by El Sapo, who then beat her up and took the bling-bling. But he did not go far - the next morning Miami Metro was already working on identifying his body. Dexter rushes to Deb, worrying about her, but she seems fine and still not in the mood to see him.
After being kicked out by his damaged sibling, Dex comes to the cabin of his suspect only to find him murdered. He still finds it hard to admit that he was so wrong, which is pinpointed by Vogel.
Debra comes to Miami Metro to answer questions about El Sapo (Quinn remembered her asking him to look that guy up once); by that time Dex has figured out she was the one who shot the bastard (go Deb!). He interrupted her "interrogation" and confronted her with this, only to realize that she's not what she used to be.
Vogel calls Dexter and says that someone's broke into her house. They find a CD with the latest "brain surgeon" murder recorded, from which it is clear that the guy Dexter suspected was just another victim of a psychopath on the loose.
So now I'm quite confident that Vogel is the murderer of the season, because:
a) She clearly knows too much about those killings
b) Like late Travis Marshall, she's the one who shows Dexter all that goodies "the bad guy" sends her: why should we believe that those parts of brain and a murder recording were delivered by someone else? She might have scooped parts of brain herself and pretend there was someone at her house easily
c) There was a perfect motivation for her to have planted that CD with videotaped murder: to prove she was right! You remember how she said that this was important for psychopaths? And undoubtedly, she's one.
But here comes the question - what's her motivation for the actual killings? She, of course, could be eliminating her former patients to avoid career issues, but hardly it was the main reason. So what else?
Well, do you remember this famous brain teaser:
A woman, while at the funeral of her own mother, met a man whom she did not know. She believed him to be her dream partner so much, that she fell in love with him right there, but never asked for his number and could not find him. A few days later she killed her sister.
Question: What was her motive for killing her sister?
I assume she used these murders to get close to Dexter, although, I'm not sure, what for.
Also, Dexter keeps showing how non-psychopathic he is: all this wanting to share his feelings, caring for his sister surprise Vogel. May it be that he was just a damaged kid, who could have never turned into a killer, providing there was a good psychiatrist instead of Vogel?