The island won't let you die until it's done with you. The candidates are all protected from death in this way. As long as they can potentially replace Jacob, the island isn't done with them. Once someone is no longer a candidate, they can easily die. Let's examine the 6 candidates. Locke, Jarrah, Kwon, Reyes, Shephard, Ford.
Locke was on a mission. To bring everyone back to the island. He then decides the love of his life, Helen, is more important and makes Abaddon bring him to her. Immediately after Locke displays that his love is more important than the island, Abaddon is shot and Locke is then killed. No longer a candidate, so the island was done with him.
Sayid shows that his love, Nadia, is more important than anything by making a deal with (arguably) the devil himself. At this point, he is no longer a candidate.
I'm guessing Jin was the Kwon candidate. He showed in the sub that his love is more important than anything. Therefore, no longer a candidate and can die.
I'm not sure what will happen to Hurley. Surely something dealing with Libby or listening to Jacob and Hurley chooses unwisely.
Jack and Sawyer will eventually have to choose between the island and saving Kate. One will choose to save Kate, the other, the island. The one who decides the island is more important than Kate becomes the new Jacob.
Obviously, most would think Jack would be the one since he is all island-crazy. However, he already showed on the sub that Kate is more important than the island by getting on it. So perhaps, Jack isn't even a candidate anymore.
I'm probably wrong, but a lot of it does tie together nicely with the whole sideways LOVE theme.