It is Debra. She has also been brainwashed by that doctor at some point. So Dexter must kill Debra :-)
Showtime (ended 2013)
It doesn't really make sense for it to be Debra. The timelines don't add up and the show isn't trying to hint that it may be possible.
It doesn't make sense that Vogel is the killer either; since she sees gifts placed outside her door. The only way that makes sense if she has split personality disorder and can't remember when she's a killer. It doesn't really explain how Vogel managed to hoist a guy on a fish hook, hoist him down and shoot him through the head. She would need help. I think vogel is too obvious as the killer and thus is the red herring.
It's probably somebody that hasn't been introduced to the audience yet.
@q666: That's certainly an interesting theory and the story telling logic makes perfect sense. There is something definitely fishy about Vogel. If she is the killer and she is controlling someone or a number of people (ex-patients), is it just to get Dexter's attention? It would certainly be a good way of re-entering Dexter's life but why now? It could be a red herring and hopefully we will find out more in the next episode. Regardless of whether she is or isn't a killer her motives aren't really that clear.
On the point about Dexter not being a psychopath, the reason Harry took him to see Vogel was because he was killing animals etc. Surely there would be a big difference between Dexer having some other problems like PTSD (from being left in a big pool of your mother's blood) and budding psychopathic/sociopathic tendencies as a small child? I'm not fully convinced on that but your tentative plot does gel together so nicely. That would be an excellent reveal.
On another point, I am quite disturbed by Vogel's blanket refusal to see Dexter's emotions as actual feelings because "it doesn't fit the model". Nothing is ever that straightforward. If you are right about Dexter not being born a psychopath she could be trying to stamp out his very real emotions because she is trying to keep him "perfect". On the other hand, she could be one of those people who thinks they are always right.
It's clearly Vogel. She's playing Dex like a fiddle! She is systematically having him eliminate those she experimented on so no one can tell the world she's a specialist on psychopaths because she is herself a psychopath. She has molded others the same way she and harry molded Dexter into a killer. She told Dex he's perfect, that's because she believes the world belongs to the psychos. If I don't learn she's been threatening herself I'll be very surprised, though it's possible I suppose that one of the kids she messed with now wants revenge. I still believe she is leading Dex, and now Deb, down a very dark road. It looks like she's gonna manipulate Deb as well, mostly because Deb is the only thing standing in Vogel's path tohave Dex under her total control. Hope he realizes before it's all too late!