I have been depressingly slow in coming to this realisation, and apologies in advance if this is old news to you at this stage, but over the past few days I have been trying to piece together the on-Island events of "The End" in an attempt to see the bigger picture, when everyone else has been debating about the nature of the flash-sideways. The reveal of the afterlife appealed to me greatly, not because I am a hugely religious person or believer in the mystical, but because the way in which the whole timeline progressed since "LA X" was pure artistic genius. This, coupled with Michael Giacchino's music accompanying the "awakenings", made the flash-sideways the most appealing "reality" in the episode. Only today, however, have I began to see the true importance of the Heart of the Island, and also what Sun, Jin and Sayid died for, something which had confounded me a bit as a result of one of Jacob's trademark cryptic explanations.
I owe this mainly to Doc Jensen of EW, who explained his reasoning behind why it was so important to mankind that the Man in Black remained on the Island, which ties in very nicely with the purgatory-like afterlife of the flash-sideways. I believe that MiB's original plan to kill the candidates would, if succeeded, allow him to destroy the Island by extinguishing the Source, given that it would be without a protector. The extinguishing of the Source would be necessary in order to release MiB of his spiritual/metaphysical bond to the Island, i.e. rendering him soulless. However, I believe that this would extend to all mankind, rendering every living thing soulless. Without a soul, how can people ascend to the flash-sideways afterlife? How can people reunite with the people most important to them and move on? The extinguishing of the Source would lead to the end of spiritual existence for mankind, and this is why it was imperative that the Man in Black remained on the Island.
Of course, Desmond extinguished the Source, albeit temporarily, which allowed this potential soulless existence to exist momentarily. Ironically, it was the Man in Black's soullessness that resulted in his downfall. He was mortal again, and thus could be killed like any other man. The fact that the Island was not destroyed means that "malevolence", "evil", "darkness" (as Jacob put it) was released on mankind only temporarily, but Jack sacrificed himself for the reignition of the Source, and ultimately, for the survival of the spirit of humanity. The Source is essentially the mysteries of life in the form of two of Mother Nature's greatest gifts; water and light. Therefore, the Island is the key to spiritual existence beyond this realm, because without the Island, our destiny would be limited only to what this physical existence has to offer.
On a sidenote, I believe that in the period in which there was no appointed protector of the light (from Jacob's death to Jack's appointment), the Source was compromised, possibly allowing some of the soullessness (I need to find a better word, suggestions please) seep into the world. Sayid was bathed in the Temple water, which more than likely originated in the Source. However, since evil was prevailing etc etc, Sayid was left without a soul, which makes sense when you think about it. I can't extend this theory to what happened to Claire, but I always got the impression that it was her association with the Man in Black that changed her personality. I don't believe she was soulless in any way.