Back in the dawn of time, when vampires were still creatures we slew instead of swooning over, Buffy the Vampire Slayer gave us the saga of Angel and his evil version, Angelus. Vampire Diaries has given us lots of vamps who oscillate between good and evil, but the main Angel/Angelus-type split was always between the nice Elena and her evil twin, Katherine. But what does it mean when Elena and Katherine are almost indistinguishable? Spoilers ahead...
In case you missed it, Elena's become a vampire — and not only that, she turned her humanity off, so she has no remorse and no compassion. She and Rebekah, the Original vampire sister, are tracking Katherine, who has the one and only dose of the cure for vampirism, which Rebekah wants to take — and which the Salvatore brothers want Elena to take.
Of course, last night's episode goes out of its way to show the differences between the two women. Elena has to impersonate Katherine, and she has a hard time capturing the Katherine sneer. She also isn't as good at playing men as Katherine, as she discovers when she finds that Katherine has twisted Elijah, one of the Original Brothers, around her little finger.
But maybe Katherine has also changed and become a nicer person, while Elena's been turning off her humanity and becoming a psycho vamp? Katherine certainly tries to claim so. She's made a pact with Elijah: he'll broker a peace between Katherine and Klaus, who's been hunting Katherine for 500 years. In return, Katherine will give Klaus the cure, which could be used against him, so Klaus can destroy it.
Katherine's deal goes South when Elijah finds out from Elena that Katherine killed Jeremy, Elena's brother — thus cutting Elena off from humanity and leaving her alone. This strikes Elijah, who's sentimental about Elena's sentimentality, as a terrible misdeed. Maybe Elijah didn't get the memo that Elena and Jeremy teamed up to kill Elijah's own brother, Kol? Or maybe he just didn't like Kol that much?
In the end, Elijah meets up with his sister and they take the cure to Klaus, without promising much of anything to Katherine — who's trying to show that she's changed by giving the cure away. And Elena tells Stefan and Damon once and for all that she doesn't want to be human, and she's not interested in reclaiming her nice, compassionate human self before it's too late to go back. To make her point, she kills a waitress and tells the Salvatore boys she's the first of many who will die until they stop trying to fix her. (Again, why was killing Jeremy so bad, in particular? The waitress that Elena kills in this episode was probably more innocent and had just as many people who would miss her.)
In any case, Elijah adds his voice to that of the Salvatores, imploring Elena to hold onto the thing that makes her special and wonderful, the thing that Katherine calls her "snowflake" quality at the start of the episode — her compassion.
So what's the difference between Elena and Katherine? Possibly that Elena is at the beginning of a process that Katherine is at the end of, a sort of hardening and closing off of possibilities. Centuries on the run have left Katherine unable to do anything but lie and scheme. But also, Elena may not have compassion but she still has a certain straightforwardness. She doesn't seem to be trying to manipulate Stefan or Damon, just telling them to fuck off if they can't accept her the way she is.
Meanwhile, Rebekah searches for the cure with Damon — and when she apparently finds it, she quaffs it. For a moment, Damon can't think of a reason to stop her, because part of him still wants vampire Elena, despite the fact that Vamp Elena is a "stone cold bitch." This lapse causes Stefan to make a resolution once and for all — he's through being stuck in the same old pattern of fighting over the same woman with his brother. Once they sort Elena out, Stefan is going to hit the road, Easy Rider-style.
And then there's the episode's other subplot — Klaus and Caroline wrangling over who's a bad person, yet again. This time, Klaus thinks he has a bit of white oak stake stuck in his back and wants Caroline's help getting it out — but it turns out to be just Silas' mind control, messing with him. The Originals are the only ones who can compel other vampires, and now Silas can compel the Originals, after a fashion. The most telling scene in the Klaus/Caroline wrangling is when Klaus points out that Tyler tried to kill him. "We've all tried to kill you," Caroline says, as if that makes it okay.
And there you have Vampire Diaries season four, in a nutshell. Almost everybody has tried to kill almost everybody else, and in many cases they've succeeded. We might decide that Katherine killing Jeremy is worse than Jeremy killing Kol, or we might decide that everybody's just equally reprehensible. What matters, in the end, is that people (and by people I mean Elena) hold on to the things that make them special.