He abused his position, broke his oath
There is also a case that could be made that he had a moral and legal obligation under international law (to which the US is subject), to expose the crimes he saw.
There is a larger debate that should be had about how much of that information really should be secret, and if so from who, and then for how long.
I think this question is already answered that most, if not all, of the information leaked by Manning should not have been secret. From what I've seen the information falls into basically two categories, either it's innocuous, or it reveals immoral and often criminal behaviour. Neither of these should have been kept secret.
Even if we assume that Manning was doing 'the right thing by [caring] about freedom of information, exposing war crimes, and holding the powerful responsible for their atrocities , his acts are those of a vigilante. Thus, his methods subvert his cause.
Calling him a vigilante is quite a stretch since he didn't really punish anyone other than exposing what they were doing.
If he did what he did and blindly uploaded to wikileaks... well then that's the end of it. He's a naive fool who thought his cause of the week was worth the risk. Maybe he still feels that way?
I would hope that preventing war crimes and exposing government wrongdoing is more than just his "cause of the week". Maybe you believe the things he exposed were just not that serious? In my opinion killing civilians should be taken very seriously, and it should be punished appropriately instead of covered up.