You surely are not a halo fan, if you believe chief is, or has died... This is oni's way of covering up that he's gone rogue. You can't have the universes most respected hero going rogue, that would destroy beliefs and how the public acts. His "death" is far more noble then his "betrayal". Hence why ONI has covered it up.
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I believe ONI has something to do with this. No way Chief is dead.
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He hasn't gone rogue if he's still doing his job.
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Possibly later in the game he goes off and does his own thing and ONI and the UNSC might not like that