I was told often as a child -- no, I breathed it -- that I should follow my conscience, do what's right, protect the weak from the strong, act on principle; that I was free and everyone should be. And since the adult world is intent on destroying children, the temptation of heroism is that much greater. I would be that hero. That pathetic child self would be redeemed.
The mistake -- Bradley Manning's mistake, it seems, one tied up with his heroism -- is believing that they meant it. The mistake is not recognizing that principle persists only because it's useful, because it redeems someone, because it helps someone manipulate. She who acts on principle will suffer at the hands of the principle pushers. She who acts to please will be rewarded.
Check out Big Bird, starting around 2:07: