Strangest way possible? No, simply the most situationally effective. If they had instead tortured and murdered the people who who talked about the torture and murder, they’d have just been proving their atrociousness.
By dressing things in the garb of criminality and law, by putting Manning into the awful oubliette that is our penal system, they create an example of how they are willing to destroy lives, but give it the veneer of legitimacy and rectitude.
Oubliette - I use that term purposefully. An oubliette is a place of forgetting. The word itself is cognate with oblivion and obliterate. And that is precisely what the government has chosen to do to Manning. Locked away, out of sight, slowly to creep out of mind, to be forgotten, to effectively remove from existence without the outcry that spilled blood receives.
What a strange, terrible, ill world we live in. Much work lies ahead to fix it.