Down at the shipping yards, LaGuerta, gun and flashlight out, enters the open container. Estrada's corpse is enough of a distraction, I guess, that's she's unprepared for Dexter to do his patented appearance out of nowhere, and she's unconscious in a heartbeat. He then reveals his plan: He gave LaGuerta a low enough dose of M99 so it won't be found in her system (oh really?), and he's going to discharge her gun into Estrada's chest right at the point of the stab wound, and then use Estrada's gun to shoot LaGuerta. I'm not sure the amount of blood Estrada has left is going to seem consistent with a shot that apparently killed him instantly, but that's probably the least of Dexter's worries at the moment...

...especially since Deb is now arriving on the scene, gun in hand. She hears a shot and rushes into the container, wherein she finds Dexter getting LaGuerta ready for the end. When he sees his sister, he tells her she shouldn't be there, and in a little-girl voice, she tells him he can't do this. As she tries to convince him, LaGuerta regains some measure of consciousness, which is pretty much the final step in the die being cast irrevocably. Hearing Deb hysterically tell Dexter that he can't do this, LaGuerta tells Deb to shoot Dexter, and she might be overplaying her hand here, but she is on elephant tranquilizers.

Dexter grabs a knife, but Deb covers him as LaGuerta keeps saying she needs to shoot Dexter. Dexter looks sadly at Deb, the knife in his hand, while LaGuerta tells her that "this" isn't who Deb is. "You're a good cop. You're a good person. You're not like him. Put him down!" Deb's confusion and desperation are palpable, and Dexter finally realizes what he's done to her as he admits everything LaGuerta said is true, and Deb's a good person. "It's okay." He's giving her leave to kill him (or at least shoot him; death doesn't have to be the result for him here), and he drops the knife. He quotes another killer -- "Do what you gotta do" -- and Deb, crying, utters his name -- and SHOOTS LAGUERTA DEAD.

Of course, you can cynically say that of course she wouldn't shoot Dexter, but the scene was constructed and acted well enough for there to be some doubt, and I give the show credit for making the stakes high enough that this outcome is believable. It's a smart progression -- at the end of last season, Deb found out Dexter was a killer; at the end of this one, she joins him. As Jennifer Carpenter continues to nail it, Deb rushes over, wailing, and takes LaGuerta's corpse in her arms, and speaking of people who did their best work this season, I'll miss Lauren Velez, but they gave her ten times more interesting stuff to do this season than in any other, so I can only imagine she's glad to go out on a high note. Dexter kneels beside them helplessly as he tries to get his head around what just happened...