We've been told for many episodes that if the Monster left The Island, the castaways and their loved ones would cease to exist. I took this to mean that if Fake Locke got away, reality would go POOF! Instead, this is how I add it up:
1. In the Lost world, people are an inextricable blend of matter and spirit.
2. Fake Locke was all spirit — an unnatural state of being. But it made him invulnerable, because spirit is indestructible.
3. To kill Fake Locke, you had to either restore him to his natural state of matter and spirit... or convert him from all spirit to all matter, which is to say, a completely mechanical animal, and thus killable.
4. The rub is that to the procedure renders everyone into mechanical animals, which is to say, devoid of a soul.
5.Without the soul, we cannot pass into the next life or into the afterlife without our community of redemption partners — the people we love.
6. Fake Locke wanted to leave The Island.
7. Fake Locke was bonded to The Island by Island magic.
8. The same procedure required to break that spell (i.e., destroying The Island) is the same procedure that would convert Fake Locke and everyone into soulless zombies incapable of having a happily ever after with our loved ones (i.e., your community of redemption partners) because we need our souls to move into the afterlife.
9. Hence: Fake Locke leaving The Island = Annihilation (when you die) for you and everyone you love.