They probably did that for a while.
Sadly, they as an agency were un-vettable to the general public, and abused that position to create tons of blatantly unconstitutional programs that they tried to hide.
There are truly evil people in this world, way worse than we probably realize. Our military is not perfect, our country is not perfect, no country or military is, but we generally do our very best to do what is right historically speaking. It's hard to see that if you get lost in the politics of things.
or there's a much simpler explanation: the awful things we do very visibly (or simply casually declassify and admit to decades later¹) are a perfectly reasonable basis to condemn basically the entire history of this country and there's no reason to believe in some sort of political dark matter that balances the moral equation.
¹ for instance, if you were right, you'd think there'd be more widely-agreed success stories coming out like this, but no, it tends to be more in the vein of "we destabilized another democratically elected government because that's not the side we think should have won". i wonder what's up with that