The Importance of the Island by Braungeo123

So I have heard things to the effect of that Jacob is basically a prison guard to MIB.

what if they both are protectors of the island, only that MIB just wants to get off the island at any price while Jacob wants to find a replacement.

Jacob cares about the future of the island given its special qualities, while MIB said that there is nothing special about the island. MIB has become frustrated over the years because Jacob is trying to find his perfect replacement before he will leave, which MIB does not think will ever happen. MIB has lost faith in the candidates and believes he will be stuck there forever if he lets Jacob find the perfect candidate to replace him (or both of them).

Because of this MIB's patience has worn thin, and he has become so fed up that he had to find a loophole to kill Jacob, and hopefully (for him) get off the island. Thats why he so nonchalantly offered Ben the power of protecting the island, at this point he does not care anymore who stays to protect the island, as long as he can get off.

Jacob has patience and believes that one of candidates will be able to replace him, while over the years MIB's patience has grown increasingly thin and he doesn't care about protecting the island anymore, he just wants to go home. He doesn't want to sit around and wait for Jacob to find a replacement. He thinks the same thing will happen this time, and they will just have to go looking for more candidates.

I think this theory has some legs, it would make sense why MIB/smoke monster seems to judge people and to some extent protect the island and act as a security system for the island. I think both Jacob and MIB are protectors of the island, but that MIB has become jaded and just wants to leave at this point, perhaps has lost his way.