LOST & DEATH

A new random thought bouncing off the inner skull lining:

Jacob is looking for a "replacement." MIB feels "trapped" to the Island and wants to go "home."
In old school corporate America, it sounds like a couple of grumpy cubical workers.

Which got me to thinking that may be the best description of them to date.

It is clear that Jacob has been manipulating people for a long time. He is not adverse to people being killed, even when he could intervene to save them. MIB as Smokey has a clear kill sheet on the Island. (Lostpedia has it down at 21).

Most of the main characters have had their own "life and death" issues. There has not been any overwrought person debating the choice of not pulling the trigger on someone else. They all appear as potential candidates to replace Jacob.

Jacob and MIB may have the same job description, they just go about it differently. Their whole purpose could be to cause humans to die, the agent that allows the circle of life to keep spinning.
The Japanese culture has a word for them, shinigami (see, Death Note.) Both Jacob and MIB, as merchants of death, want to leave their positions. Jacob wants a replacement, MIB no longer cares.