The Chief has no political power. He's not a leader. I don't see how he can become a tyrant.

I just read "The Lost Fleet" series, and I'm going to bring it up for contrast. In The Lost Fleet, a commander who fought a desperate rear defense in the opening act of a long-running interstellar war is posthumously built up as a hero by his own government. Time wears away the details, until Captain (Promoted, again posthumously) 'Blackjack' Geary is synonymous with God. Politicians and fleet officers alike argue over the path to victory by appealing to what Blackjack would have done. Many citizens of the Alliance truly believe that Blackjack Geary is among the Living Stars, and will return to help the Alliance in its hour of need.

A century later (I did say it was a long war, didn't I) they find his escape pod, dust off the cryogenics, and inform him that he's now in charge of the largest fleet ever assembled by the Alliance. Admiral and a bunch of other senior officers got killed, and he's got seniority by date of rank.

Throughout the rest of the series, there is this real, palpable fear that Captain Geary is going to usurp the Alliance and establish a dictatorship the minute he gets the fleet back home. For some, it's not fear but hope that the infallible Blackjack Geary will pack out the deadwood and force the politicians to rule justly.

On one hand, we have Blackjack. On the other, we have the Master Chief.
One is a venerated hero with heaps of both political power and firepower at his command, and some pointed opinions about the rule of law. The other is a windup tin soldier. A modern Golem, basically. You can't make a leader out of that.

I really don't see why ONI would go to the trouble of smearing the Chief as a traitor unless he's done something to deserve it.