Boom!!! I Solved Lost by Wolv256

Both Jacob and the Man in Black (Smoke Monster) are Egyptain gods. Jacob is Set, the smoke monster is Apep, Richard Alpert is Ra, and I think Ilana is Taweret. The smoke monster believes that people are selfish, unredeemable, and should be wiped out. Jacob thinks people can be good and tries to protect and redeem them. As per Egyptain mythology, the smoke monster (Apep) is bound by Ash (who was also an Egyptian god), which is why he can't cross the ring of ash.

But, the important thing is that both Jacob and the smoke monster can "claim" souls when they die, as with what happened to Sayid. When someone is "claimed" they can come back from the dead, but can be controlled and taken over whenever the gods feel like it. This is the "sickness" that happened to Rousseau's crew, they were claimed by the smoke monster.

Aaron (Claire's baby) died in the plane crash. She complained about not being able to feel the baby after the crash. Then the smoke monster "claimed" him and brought him back to life (while still in the womb), under his control. This essentially makes him the antichrist, in that he's controlled by an evil being that wants to wipe out humanity. If Aaron leaves the island and grows up, that's what he'll do.

All the talk of events that are meant to "Save the world" are in fact events that are stopping the smoke monster from leaving the island and getting unleashed.

Walt was kidnapped because Jacob told the Others to get the kid because he knew the smoke monster was using him. However, the Others misunderstood and grabbed Walt instead of Aaron.

However, Jacob had a loophole as well. When Locke was pushed out of the building by his dad, he died. Jacob "claimed" him when he touched him. All the times Locke seemed to test people or be someone else, it's when Jacob was taking over. So even though Ben killed Locke, he was really already dead, or undead actually, and just played along with it to get back to the island. So, even though Locke is dead, he'll come back to life because he's claimed by Jacob. However, he's buried. So he'll have to tell Hurley (who can talk to dead people) to dig him back up, so he can come back to life and lead them to change the past and undo the things that led to his death, the plane crashing, and the possessed Baby Aaron destroying the world.

If you've ever heard the writers/producers Damon Lindelof and Carlton Cuse talk they always joke that when they run out of ideas and "jump the shark" that the last season of Lost will by the "Zombie Season". They use this to respond to people who say they don't know what they're doing and just randomly bring people back. But, I think it's really an inside joke to them that it's kinda how they are going to end it. Locke coming back from the dead also kinda parallels Jesus, in how they like referencing religions (Jack's Dad is named Christian). It'll end up with Locke facing Locke, as Jacob and the smoke monster have each taking his shape.