What a powerful episode this was! We discover that "The Candidate" was a trick title. This episode does not reveal who the final candidate to replace Jacob is (I think it's clearly Jack), though it does eliminate three other candidates.
The title refers to Locke in the flash-sideways being a candidate for an experimental spinal surgery. This becomes the center for discussion of this episode's main theme, "letting go." Locke refuses the surgery because he cannot forgive himself for turning his father into a vegetable.
It is interesting that Jack is now the man of faith in both timelines. Each timeline invokes Locke's former belief. Jack says, "I wish you believed me," as Locke wheels away after refusing the surgery. In the Island timeline, Jack tells the Man in Black that it was John Locke who told him not to leave the Island.
They attempt to defy the Man in Black by getting on the sub without him, and inadvertently fall into his trap. In a reversal of his usual style of reactionary leadership, Jack implores Sawyer to leave the bomb alone. He reasons that the Man in Black needs them dead, but is "not allowed" to kill them himself. By messing with the bomb, Sawyer takes on the responsibility for killing them.
It was, however, Sayid's idea to pull the wires. Understanding that Sawyer's actions have placed them in jeopardy, Sayid carries the bomb as far away from them as he can. He reveals his goodness by sacrificing himself for them.
The explosion gives them an escape route, but injures Sawyer and pins Sun behind some rubble. Unwilling to let go of his wife but unable to free her, Jin drowns with Sun—their hands clasped to the end.
And at last, the story confirms that the Man in Black is the greater evil between him and Widmore. Despite his callous attitude, Widmore is attempting to protect them—a plan which only fails because Sayid apparently does not feel the pain of the sonic fence and was therefore able to shut the power down.
The quality of character demonstrated by the candidates seems to be the only thing capable of foiling the Man in Black's plans. Ultimately, the candidate who replaces Jacob will be the one who stands on principles (no pun intended) and refuses to be corrupted.
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