At the end of Ab Aeterno, Jacob says to MIB, "I see you got my present," to which MIB replies, "Don't gloat Jacob. It doesn't consume you."
What the hell is he talking about???
If MIB is the one that supposedly posses the "darkness" that consumes people once they are infected (Claire, Sayid), isn't it ironic that he would tell Jacob something "doesn't consume you"? After seeing Jacob corrupt himself by appointing Richard to speak on his belhalf to the people of the island, and after hearing MIB say "it" doesn't consume you, a strong argument can be made that MIB is the one who believes in free will, while Jacob doesn't believe that people can make the right decisions on their own.
And on a side note, maybe MIB told Richard the truth when he said that the devil took his human form. What if the man we are seeing as MIB now was once Jacob, and the devil took his human form to appear as the "good guy." It would make sense that MIB would want to kill Jacob for not only keeping him on the island but also taking his body. And perhaps MIB couldn't kill Jacob because he would, in a sense, be killing himself... until he found a new body to host his spirit.