Locke is and always has been MIB - they are one and the same. Back in one of Locke's flashbacks, it was said that Locke was emmacuately conceived - this seems to not have been true since Anthony Cooper was introduced as Locke's conman dad, but what if Cooper was just bluffing all along and Locke was born magically? i think that Jacob somehow killed MIB, but MIB's consciousness was transfered into Locke's. This explains Locke's mysterious knowledge of things back in the early seasons - his drawing of the smoke monster as a child, his knowledge of the weather, etc. - familiar surroundings brought back dormant memories. Once Locke 'died' and came back to the island, the electromagnetism of the island ressurected MIB, who now remembers everything. That is why Locke says to Ben that Jacob and him met 'in a manner of speaking' - since MIB has never really met Jacob as Locke, except that one time when Jacob touched him ! after his fall. If you look at that episode, Locke has a really strange look on his face as that happened, as though he knew Jacob already. This also accounts for Locke making promises to Sun to help her with Jin, and telling her that he is the same man he always was - because he is, and he truly wants to help her as he grew fond of her and the other castaways during his time as Locke.
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