There are a bunch of theories on here about Jacob and Smokey/MIB. Here is me borrowing some from them and adding a bit of my own to it:
Jacob is the light/good force on the island. Smokey/MIB is the dark/evil force on the island. This may not be correct because it seems obvious, and LOST is noted for the many plot twists that occur during all of the previous seasons.
For some reason, it seems that Smokey/MIB wants to "go home" which is the one thing he can't do. Or at least he couldn't do until Jacob (as the blonde haired man) was killed. Somehow, I believe that Jacob has the ability to keep Smokey/MIB on the island. I am not sure how, but Smokey wanted Jacob dead for a long time (Season 5 finale), and I find it hard to believe that it was only because Jacob was seemingly brining people to the island. I can only imagine it was because Jacob was keeping Smokey/MIB on the island with some force we have yet to see.
Now this would make Jacob appear to be the bad guy, but it's not so. I believe the reason Smokey/MIB wants to get off the island is to destroy humanity. He doesn't seem to have any qualms about killing people, as he has been doing so since Season 1 until the present. What reason he has for destroying humanity is not yet known to me, nor do I have any theories about it thus far. The only thing that comes to mind is Alfred from the Dark Knight when he says "Some men just want to watch the world burn." Perhaps Smokey/MIB is upset because he has been trapped on the island too long. Who knows.