Lost has been making us question who the 'good guys' throughout it's entire run. Every person interested in the island always believes they are in the right. First it was the Others, then the DHARMA Initiative, both believing they were the 'good guys'.
Now we have Jacob and the MIB, and though it's easy to see one as good and one as evil due to the black/white=good/evil symbols, the show has made a point of showing that Jacob can be just manipulative as the MIB.
Jacob's main difference with the MIB is that he believes people have the chance to be good, whereas the MIB believes that people are inherently destructive. The MIB manipulates people into joining his cause, and is not afraid to kill people when he needs to. This seems to make him evil, but Jacob has also manipulated all of the 815ers into coming to the island, knowing the many of the people involved would be killed in the process, an act that, however reasonable, is definitely not 'good'.
So although Jacob and the MIB have been positioned as being the embodiment of good and evil, repectively, I don't think that's what the literally are, at least not originally. This season, the MIB has dropped little tidbits of info about his past with Jacob. It seems like their history extends outside of the island, much like all of the other characters. Were they brothers or something? The MIB mentioned something once about how Jacob killed, or let die someone who was close to him.(I think I heard something to that effect. Correct me if I'm wrong)
Anyways, somehow they arrive on the island.(whether it's an accident or otherwise)They found where the 'power' or whatever is held. They are warned somehow against trying to take the power, but the MIB has lost someone close to him,(much like other Lost characters) and he wants her/him back. So he takes the power. Or, to phrase it differently, he opens Pandora's Box. He then becomes the embodiment of the 'evil' in the box, thus the black smoke and so on. Jacob then takes what's left in the box: hope.
Although they have both gained superhuman abilities, they are still human, and not gods, which Jacob makes clear when he tells Richard that he cannot absolve him of his sins. The MIB wants to leave the island, to continue his life, but Jacob doesn't want to allow the the 'evil' in him to reach outside of the island, so he contains him there. And setting everything else into motion. --Johnknolling 21:08, March 25, 2010 (UTC)