After six seasons of watching Dexter kill people, it's hard to believe that he might be able to cure himself simply by abstaining while remaining under the watchful eye of his sister. But that's the plan Deb has in place for him now and as we saw at the start of tonight's episode, it's begun to take its toll on Dexter.
Things seem to be working in Dexter's favor right now. Sure, he has Deb babysitting him and it's caused his tension level to spike, but as we saw by the end of the episode, Deb is starting to see the situation for what it is, and she might not be able to deny the value of Dexter's code. When a scary horn-wearing meathead roams free because the proper procedures must be enforced, Dexter's quick-and-dirty approach to taking a murderer off the streets probably seems a bit less terrible.
At the start of the episode, Dexter was daydreaming about slashing throats and neck-stabbing Masuka! No Dexter! Bad Dexter! Just a fantasy. With Deb's watchful eye on him, Dexter's dark passenger was kept on a leash and it was wearing him down. The tension was so high, he ended up grabbing a guy by the throat during a cheek-swab. The guy was egging him on, but still - bad form, Dexter. Deb put a stop to the situation and after that, was willing to give Dexter a bit more space. Off his leash (somewhat), Dexter began the hunt, tracking down and stalking Speltzer, a parolee who did time for assault and was linked to a bunch of dead girls but never convicted of killing them.
Deb was on Dexter the whole time he was stalking Speltzer, and during that time, Dexter tried to convince her that this is what he does. He has a "lizard brain" and a heightened sense of danger, thanks to Harry's training. He can tell Speltzer's preparing to kill again by watching his behavior. Deb tries to take the proper channels to keeping an eye on Speltzer and that led her right to the scene of a murder in progress. On his end of things, Dexter had the evidence he needed to go after Speltzer too and managed to get there just in time to save Deb from being bludgeoned to death by the muscled murderer, who likes to blast metal music, chase his victim through a messy house and beat her to death.
Speltzer got away, the girl he was killing was dead, and now Deb sees. Well, she kind of sees. One incident isn't going to convince her that it's ok to be a serial killer. She made the connection between the blood slides and Speltzer's trophies and realized Dexter collects trophies because he liked to kill. Um… yeah Deb, he's a serial killer. While I think she should've made that connection much sooner, I do think it's relevant that Deb sees this (and that we see Deb see this).
Sometimes I hear people describe Dexter as a vigilante when they're talking about this series, but that seems wrong to me. Dexter doesn't kill bad guys because he wants to right wrongs or deliver justice. He kills them because he likes to kill people. He just happens to channel that through Harry's code. It's what works. It's how he can be what he is and live with himself. And it's also kept him from getting caught.
Deb needed to understand that, and now we know she does. So if she decides to turn a blind eye to what he's doing, it won't be because she's under the delusion that her brother is some kind of superhero. He's a murderer. But he murders murderers and he's her brother and the man she is or thought she was in love with. Maybe she'll be able to make peace with that somehow. At the very least, she might be starting to see the value in what Dexter does. Things ended tonight with her deciding she didn't want to live with Dexter anymore. But nothing is settled.