As for your comments about the proper chain of command, those are the very people that are outed by these leaks. Telling Al Capone that he's doing something questionable to his face is imprudent. Catch-22.
Why do you assume he would go through his own chain of command?
Proper channels for him to use as a military whistleblower would include anything from tipping off CID/NCIS/OSI, to contacting his Congressman (or other high level government official), and everything in between.
Manning had quite a few options, but chose to betray his country out of cowardice.