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When Walt appears to Locke in season 3, he warns him not to shoot himself. Then he tells him that he has work to do. Locke responds by killing Naomi before she can send a message to her people. He later attempts to prevent Jack from communicating with Naomi's people. Since Walt appears much older and the writers promised an explanation, we can assume that he is speaking from the future where he is older - possibly the future (or alternate future) seen during the last few minutes of the season 3 finale.
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