Narrative-wise, I think the reason Eko died has to do with having his closure about the life he had led before the island. Though he was a man of his Christian faith, Eko-on-island finally refused the guilt that was with him through his whole life. He saw his life under a different light and realized that he did what he had to do to survive, may it be stealing food for Yami or drug dealing. That seemed to me to be his personal revelation in front of Smokie before the monster killed him.

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"Bear...? Is that you?"-- Hurley S03E4