Unchecked, indiscriminate killing of civilians in Iraq. Proving that the military was over there operating outside the spirit of what we as a supposed free democratic and just society stand for.
Most of the leaked information didn't concern that though. The most damning leak was the Collateral Murder video, and at most, it's more an example of negligence or confusion over the rules of engagement than indiscriminate killing of civilians.
Pretty damn simple.