Let us presume that all the LOST characters from Flight 815 survived the plane crash and were "alive" on the island. They were all "saved" because a mysterious supernatural being named Jacob needed one of them to man-up and become the island successor guardian - - - to "kill" the MIB (smoke monster) because for no apparent reason, Jacob could not kill it because it was against the rules.
Rules, mind you, that Jacob broke by actually killing his brother after he killed their Crazy Mother. Jacob's action of beating his brother and throwing his body into the light cave apparently killed him and turned whatever spirit or soul into a violent smoke monster. Jacob took his dead brother and Crazy Mother and buried them in the water cave (Jack found them and we knew the bodies as Adam and Eve).
So the whole plot line reasoning about Jacob needing a successor was totally flawed. He was an immortal being, so how could he actually die? And when he did die by being stabbed by Ben in the Tawaret statue, he apparently "lived on" like a smoke monster creating images of himself as a young boy to torment MIB/Flocke, steal back his ashes from Hurley, and making his big pep rally speech to his remaining candidates around his last campfire.
Near the end, Sawyer asks jack why Locke didn't just kill
Desmond. Jack suggests maybe it was one of his 'rules'. Sawyer suggests
that he himself was responsible for the deaths on the submarine, because he
attempted to defuse the bomb against Jack's advice. Jack insists that Flocke killed them. Behind them, Hurley is confronted by young Jacob who asks for the ashes, then snatches them and runs off. Hurley chases him and comes across adult Jacob
seated by a fire. Jacob tells Hurley that the ashes are in the fire and
that when the fire goes out, he will not be seen again, adding "We are
very close to the end."
Why Jacob needs to be a boy to get his ashes back is strange, since Hurley knows of the adult Jacob from the lighthouse. So there is no need for another level of confusion.
As night falls, Hurley leads Kate, Sawyer and Jack to Jacob's
fire. Jacob greets them by their first names. Hurley is surprised that
they can all see Jacob. Kate asks Jacob whether he is the one who wrote
the names on the wall, and whether it is their candidacy that ultimately
led to their deaths. She also demands to know that Sun, Jin, and Sayid
didn't die for nothing. Jacob says he will tell the group what they died
for and why he chose them. He adds that by the time the fire is out one
of them will have to take his place as protector of the Island.
Jacob explains that a very long time ago he made a mistake, and as a
result there is a good chance that everyone is going to die. He
acknowledges that he is responsible for the current state of the Man in
Black. The Monster has been trying to kill him and that when it
succeeded, someone would have to replace him: that is why he brought
them all to the Island. Challenged by Sawyer, Jacob explains that he didn't drag anyone out of a
happy existence but that they were all flawed. He says that he chose
them because they were all like him - all alone, all looking for
something that they couldn't find. He says he chose them because they
needed the Island as much as the Island needed them. Jacob tells Kate
her name was crossed off because she became a mother, but that she is
not disqualified. He explains that the task for the candidate is to protect the light at the center of the Island.
In essence, there were five candidates left on the island: Jack, Hurley, Sawyer, Kate and Ben. Austin and Linus had their names crossed off in the cave. Why Ben was no longer a candidate is never explained; he seemed to be the most prepared and most willing to take charge. But could a guardian killer become the guardian of the island? It would appear that is how Crazy Mother operated when she murdered Claudia to take her children. Ben did the same thing by kidnapping Alex from her natural mother. We must remember that the guardians are not high moral beings. In fact, they are really self-centered, narcissistic demi-gods. From that perspective, Jack, Hurley, Sawyer, Kate and Ben each in their own way "qualified" for the guardianship job.
Jacob never explains why the light source needs to be protected, or be protected from Flocke. His statements that the candidates friends deaths were in furtherance of a plan to find a new guardian ring hollow. If MIB wanted to kill Jacob, he could just kill him (since Crazy Mom's rule was that the brothers could not harm each other, which was false). Jacob could avoided being killed by NOT bringing anyone to the island who could be corrupted into killing him (as what happened with Ben). So in reality, Jacob continued to lie to his final candidates. He wanted to bring people to the island so he would "die."
Jacob then says that they must do what he couldn't do: kill MIB (Flocke). Jack asks whether that is even possible and Jacob says that he hopes so because "he" is certainly going to try to kill them. Jacob offers the remaining candidates
a choice of who will take his place - Jack accepts, acknowledging that
he is on the island for this very purpose. Jacob asks Jack to affirm
this decision, and is pleased when he does.
Again, if Jacob is a supernatural being with full knowledge of all the island's properties, he is lying to Jack on whether MIB/Flocke is a being capable of being killed. Jacob would know. And this elaborate ritual is merely a mask to get these lost souls to buy into the dirty deed: MIB/Flocke must die.
So why would Jacob hijack human beings to come to an island that contains the entire life spirit of the universe? If one looks closely at the dark side of these proceedings, it is because it was Jacob, not MIB, who was trapped on the island. MIB was a spiritual security system that kept Jacob was leaving the island realm; he was the jailor and Jacob was imprisoned (for killing his brother perhaps?) But we saw that Jacob left the island to "touch" his candidates. But perhaps that was an illusion (or the candidates were dead but hijacked on their way to the after life).
So it came down to five people. Jacob only needed to convince, con or manipulate one person to kill Flocke. The motivation was simple: Flocke killed you friends and he will kill you. If he kills you, he will destroy the world. Heavy stuff. But if you recall, people have tried to kill Flocke but nothing harmed it. There is no magic switch to turn MIB into a mortal human. The campfire going out to end Jacob's time was a ruse because when Ilana found the ash pit, it was already extinguished so by common sense, Jacob's life was extinguished as well. But that is not the case as he continued to appear to the candidates.
It seems almost absurdly stupid. It is the make-believe aspect of the explanation that makes one recall two little kids playing pretend WAR in their back yard. Both shoot each other with toy guns, and one has their dramatic "you got me" dying moment. Then they get up and do it all over again. We have no proof that either Jacob or MIB "died" as a result any event on the island. We were just swept away into the sideways vortex.
But then again, in the context of the end, was the island conflict all that important? There appears to be faulty logic to try to bridge the island events to the fantasy of the sideways world. For if the island was the "most important" part of each of the church members lives, why not did they not all "reunited" on the island and live FOREVER together? It would seem that the island was no longer important (even though we were told that it was the source of life, death and rebirth). The island was merely closed like a bad diner going out of business.