Don't know how you all feel about Locke, but I think that he has been either "possessed" or "impersonated" since the first time he landed on the island on 815. When we met Locke, and in all of his flashbacks, he was a wimpy, needy, unconfident man who needs love, validation, consumed with jealousy and self-pity...no motivation for love (Helen) or to stick up for himself (with his father). Then, he gets to the island and all of a sudden he changes into this tough, all-knowing superman who can walk and has physical strength, hunts, knows all about backgammon (symbolism for the black/white good/evil war)?
Then don't you think it's funny how as soon as Locke turns the wheel and goes back to normalcy to recruit everyone how he turns right back into that pitiful character, crying because he can't lead, can't convince anyone, nobody listens...so I will kill myself?? The Locke of the island would not give up that easily, and not kill himself, and not cry about it.
I think there have been two lockes from the very beginning, perhaps when Jacob "touched" locke after falling out of the window, he awakened something in him..giving him the confidence to go on the walkabout etc...maybe not.
It could also explain Richard's confusion as to why when he visited locke as a child, he didn't detect anything spectacular or special about him. I think that Locke is a vessel for that dude in black at the beginning of the finale and he grabbed Locke as soon as he could do train him to do his dirty work.
I think that the Locke in the coffin IS the original Locke from the "real" world, and that the Locke fighting Jacob, was a different version of Locke from the very beginning of the show.
Thoughts?
Sarah