So, the dude who's cribbing Locke's look (you know, that guy that Jacob talked to in the first scene. I shall call him NotLocke) is the Smoke Monster. Here me out: Jacob and the smoke monster are two different forces of the island. Jacob is the good force and the Smoke Monster is the evil force. Jacob asks you to do things for him, seemingly to benefit the island. The Smoke Monster just likes killing people (RIP Mr. Eko!!! Paola and Nikki...not so much).
So, my thought is that this NotLocke dude lost his body at some point and became the Smoke Monster and because of this, for some reason, he is able to manipulate the dead (Christian Shephard, Locke, Yemi, all the spiders people stepped on, Kate's horse (is that horse dead?)). Also, remember, he's evil, so he wants to kill everyone and rid the island of all the outsiders, because that's what evil does.
Now, Locke, the lovable idiot he is, totally got duped by NotLocke. Think about it. When NotLocke was pretending to be Christian Shephard, he told Locke he had to die. When NotLocke was busting through the forest with Richard Alpert, he got Richard to tell Locke he had to die. Of course Locke was going to believe all that claptrap! Dude had a major hard-on for the island! And if the island said he had to die, he was willing to die. So, Locke dies, and the evil Smoke Monster NotLocke dude can...steal his, um, image (?) and sucker Ben into killing Jacob. Wait, how does he sucker Ben, you ask? Because he's the motherfucking Smoke Monster! And he came up to Ben all dressed up as Alex and told him he had to do what Locke said. So, basically, NotLocke told Ben to follow Not Locke! It's like if Superman told you that you had to do whatever Clark Kent said or else he'll zap your ass with Kryptonite eye beams. Get it?
Alright, all in all, good show. Sadly, Kate did not die (but she let Juliet bite it...bitch). Happily we saw what was up with Rose and Bernard (if you didn't tear up a little, then your heart is a cold dead rock of unlove), and I guess we all sat around and Googled "Egyptian crocodile god" for most of the night.