At the dinner, Vogel said that the best hobby is the one that brings you furthest from your primary occupation. Hanna fits that description, as Vogel pointed out, by "creating life". Dexter fits it too, by solving murders as opposed to killing. When asked what drove Vogel to her job, she gets quiet and states that it's something that happened long ago that she wouldn't want to bore them with.
Like someone said earlier, it's possible it's her that's the killer. Personally, I don't think it's her now that I think about it more. I think what's driving her is a relative of hers, like her son or first patient. She's fascinated by serial killers, and have dedicated her entire life not to "cure" them, but by "saving" them. She probably feel like she's responsible for whatever happened to said person, and has been making up for it by trying to help other serial killers. Dexter fascinates her because he's her greatest success story, and she wanted to see if it would work again on another killer - hence Zack. However, the real killer probably dislikes her for whatever happened between them, and killed Zack for whatever reason. Perhaps jealousy?
I'm going to throw out a theory and say that it's an old boyfriend or friend of hers, as she seem absolutely perplexed over Dexter and Hanna's affection towards each other, something she thought would be otherwise impossible. I'm guessing the brain thing could be that she used to have a relationship with someone, and that part of the brain is of special meaning to the both of them, which is probably why she pointed it out to the doctors in the original "brain surgeon" killer, because of what she learned about that part of the brain with said previous person. It's possible she's not aware that said person is alive, she might very well think he's behind bars or dead, or simply forgot all about him. Hell, she might even be the reason for it. She might have found out that said person killed someone and reported it to the police, and later felt bad about it and decided to focus on trying to save them instead. Would explain why she's fascinated about Dexter's connection to others.
It makes no sense that she'd go through the effort to have Dexter teach Zack, only to turn around and kill him. Although I'm probably giving the writers far too much credit, might very well be her.