Man in Black is Dead by John Doe

I know this will sound a little bit sci-fi, rather than a sound theory, but the nature of the smoke monster aka MIB is such that it is a sci-fi itself. So I think that MIB is dead and here is why: First of all he says himself he is dead to sayer, and when he is first found on the beach he says that he remembers dying.I think previously this phase was attributed to John Locke. MIB doesn't attempt too much to impersonate himself with someone else when he doesn't have to and he generally doesn't lie in the rest of his conversations, so it is reasonably to assume that when he says he is dead, HE is dead. This would explain several things as well as the nature of Smoke Monster - it is some sort of ghost, who died but somehow didn't go to the heaven or hell and is tuck in limbo which is an Island. Very much like in all the movies about haunted houses, except it is an island and not a house. Just like "conventional" gho! sts it can take any shape it wants, move things around, harm people, etc. and when it has no shape it exists as a pillar of smoke - ghost.

This is a reason why he has access to the memories of the dead people - because he is dead himself and can probably "meet" them once they die. This also explains why Jacob died when Ben stabbed him and the smoke monster did not when he was shot at. Jacob was alive and died ( although I think there is still a possiblity the knife was special and Jacob couldn't been killed the conventional way). However, when bullet hit MIB it went right through with no harm, because he is already dead and is just a pillar of smoke in whatever shape. That's why when he tells Sawyer that he can put a bullet in his head and see what will happen he knows very well that nothing will happen.

The MIB wants to get out of his haunted "limbo" situation and somehow impersonates Locke. When he does that he becomes more alive than dead, so he cannot change shapes anymore, but not fully alive, because he is still stuck on the Island and still can change into the pillar of smoke. I'm guessing that if he'll be able to leave the Island he'll become fully alive, that's his goal. Also I think somehow when it is in the smoke monster shape it is more dead than alive and that's why he changes into the smoke monster at the statue - not be harmed. However, when he is in Locke's shape he is more alive than dead, because he has the limitations of a body of the living.