Jacob and MIB by Lostfan4ever

This is my first attempt at a theory here. It is clear from the spoiler posts that we will get a backstory on these two. I am suggesting that Jacob and MIB have always been on the island - in fact may have even been born there - at the very least "brought" there. They are brothers- both of them candidates. Jacob fully believes in the job he has been given (to protect the island) and his brother does not - he is the skeptic - he doesn't believe in this charge that has been placed upon them. He is more selfish while Jacob is more selfless. They both want to return to their lives and need a candidate capable of watching over the island so they can leave - hence the discussion between the two on the beach in last season's "The Incident". MIB has grown tired of Jacob's search and in fact has grown tired of him feeling that he doesn't need him to remain on the island or the "loophole" is finding a way around the! rules that keep MIB on the island and killing Jacob was that loophole or part of the loophole. MIB's plan was not to just kill Jacob, but to kill him providing MIB his avenue for leaving the island.

I'm throwing this out there because if MIB was truly "evil incarnate" why wouldn't he have simply killed everyone on the island in the same manner in which he used Ben to kill Jacob? Why only hurt "the ones who won't listen" as he stated to Claire?

So, ultimately, I believe that what we will be left with on the island is Jack, in place of Jacob, Sawyer in place of MIB both of whom have the same relationship as MIB and Jacob did.