Thought I would address something that has been discussed before but not many theories as to why it was said.
- 1 reason why someone would want to kill another
A. Revenge B. Anger or Rage C. Envious or Jealousy D. Out of Despair
The context of the man in blacks comment did not seem to be out of anger or rage nor jealousy but very possibly revenge or despair. If the motive were despair, we have not seen what could possibly be bringing him to think that killing Jacob is the only way to prevent or change something that seems so critical to the man in black that he is willing to commit murder.
Now my observation of the statement made by the MIB, is of a revenge in nature. Revenge for what, we ask? Lets reflect on the past episodes and think of any occurrences and or happenings that could possibly be related to motive for revenge - AND COMMENT ON IT HERE. I have a short list of motives that may help but your observations are welcome and needed. I am only going to focus on things that are unrelated to the survivors of 815 because I think that whatever is playing out here has been going on long before the crash of Ajira flight 815.
Possible Motives: 1. Widmore of course was exiled from the Island. - Possible but I think it goes back much further than that. 2. Birthright stolen, we know who that would be of course! - I think unlikely 3. 2 immortal gods wrestling over control of humanity but unable to kill one another because of immortality. - Not sure it fits 4. Time travelers stuck in an alternate time line with one wanting to return and the other not and he is causing things to happen to prevent course correction. - Hmmm! no proof 5. One entity standing guard over the other preventing it from entering the mortal realm, in doing so, making the Island its prison thus it is only capable of causing havoc on the island. - The cabin and ashes could be a clue to this possibility. 6. Hurley was the only survivor and all of the characters and happenings are figments of his imagination. - Very possible but have the directors assurance that is not it.
7. PUT YOUR POSSIBLE MOTIVES IN COMMENTS TO THIS POST AND WE WILL DISCUSS THEM - - - - - - -