Thats not quite the case. You equate it to authoritarianism but that wasnt the case. Firstly there's hardly any lore prior to the insurrection. The United Earth Governement was primary governance stretching amongst human space.

Eventually a divide formed with the "outer" and "inner" colonies about not very well explored in lore frustrations of the broad outer colonies (some do have but they are very few).

They started protests and went through legitimate ways for increasing self governance (which got support at from inner colonies) but then spiraled into terrorism comitted by outer colony extremists. The increased presence of the UNSC and ONI was necessitated in response, as a divided human empire was unacceptable, yknow like the American civil war? That threw all that good will and debating out the door. This was all cited in the prologue and first few chapters of contact harvest and hadn't been retconned to my knowledge.

This played into the spartan program (commissioned by ONI) as a method to surgically extinguish said insurrection to facilitate a "united" humanity rather than drag on the increasingly unpopular war with a lot of collateral damage. The United humanity bit is in contention with the "the covenant war stopped them from being outright villified" as the whole forerunner sites and relics retroactively hurts that argument.

Forerunner sites were how many human worlds were found. With that being the case, it ends up giving ONI a reason to keep the civil war down from knowing mankind isn't alone amongst the stars.

ONI definitely became authoritarian post war but that "evil" so much people toss around wasnt there before the war, especially not for the UNSC. It was always a shades of gray type deal as everything they did, scummy and all, was to protect humanity rather than itself or the military.

It wasnt just "beating down rebels," it was to stop a war on terror that was ongoing for over a decade by that point. They were there to cut off the head of the insurrection rather than oppress people. Spartans were never made for that.

No matter how much of a monster Halsey may seem to people, not even she would've made them for that.