MIB is NOT Smokey... just another dead body used to further its cause...
MIB is Smokey though. If it is impossible for Jacob to kill his brother, then how could he have died at the cave? He didn't. I said this on another post, but I'll repeat it here, just because we see MIB's body doesn't mean MIB is dead. His life just changed forms, much like a caterpillar turning into a butterfly. The caterpillar doesn't die, it just changes form.
I am more feeling after thinking about last night's episode if the whole MIB = Smokie warrants more questioning. We saw the smoke only after MIB was pushed into the light - into the place that held something worse than death (according to fake mom). I made the assumption that the smoke monster was MIB - somehow changed by his presence in the light - we even saw his "discarded shell" of human form left behind. The smoke monster can take the shape of other corpses: Jack's father, and Locke. But is Smokie still MIB at heart?
Jacob released the smoke monster on the island through his evil/bad/wrong actions. Now Jacob must keep him contained on the island AND still keep people from finding the light. However, Jacob brings people to the island - presumable as potential candidates to replace him - or to play his game with Smokie who for the purposes here "is" MIB? It seems to me keeping Smokie locked away, and keeping people from finding the light would be much easier if Jacob would not bring people to the island.
I take all this to mean that Jacob's job sucks - he wants to stop doing it - he would be as thankful as was Fake mom ("thank you") if he could end it. Here is the twist: when MIB says he badly wants to kill Jacob, we assumed that was an evil act. Instead, couldn't it be that releasing him from the burden of guardian would be welcome as it was for FOM? Killing Jacob is the final act of kindness he can perform - an act precluded by the covenant formed with Jacob and Fake mom with the wine.