The Man in Black is Jacob's Prisoner by Ricardus Alpert

After thinking about it for a while, I think I've been able to combine what we knew about The Monster before S5 and the little we know about MIB, and make sense of it all. Of course, if you think I'm rambling and not making any sense, please feel free to call me on it.

Go back to S1. Back then, when we know very next to nothing about it, The Monster was called a "Security System", that protects the island. Given how I believe MIB is The Smoke Monster, that's exactly what he is, IMO- A Security system. That's something that we had all taken for granted about The Monster since S1. But there's a question we have all overlooked, which I believe to be critical- Does The Monster perform those duties willingly?

Well, after thinking about it for a while, I think the answer is no.

Think back to the scene we saw in "The Incident". Given how tired MIB seems to be of Jacob's games, why doesn't he try to act against him? The obvious answer is that "He can't kill him", but it at this point, I think he can't act against him at all. I think the most simple answer to both why MIB can't kill Jacob and can't act against him is that MIB is literally bound to serve Jacob. I'm not sure why, or by whom, but MIB has been Jacob's eternal servant and weapon as The Smoke Monster, until he found the loophole which allowed him to break free from Jacob's control.

That would explain to me the whole issue of the Summoning. Why would a being as powerful as Jacob's Nemesis respond to the summoning of the people who worship Jacob? Because he's forced to, which is also why he guards The Temple, which Jacob's people reside in.

I believe The Man in Black was cast out from whatever society he and Jacob came from. The island is his prison, with Jacob watching over him as a "Warden." Jacob has been bringing people to the island in an effort to change The Man in Black's views on humanity, and save them from destruction. That's why Jacob is called "He who will Save us All."

I have no doubt that once he's free of Jacob, MIB plans to attack the outside world. Imagine how terrifying that could be.