Someone else here reminded me that Manning actually delivered these documents to others, who WERE supposed to try to separate that out. But somebody goofed. So I'm not sure that can honestly be blamed on Manning, who actually did make an effort to expose wrongdoing while not releasing those other things to the public.
That's kind of a huge abdication of responsibility on his part, don't you think? Ultimately Manning was the person responsible for leaking classified information - it was his decision alone, and only he had the necessary access. If he really thought that the public would benefit from some of the material he released, it was his duty to separate it out.
I still think this points to naivete rather than malice, and I certainly don't buy the argument that Manning aided his enemies, which would criminalize just about any action which simply makes the US look bad. But I still find Manning's behavior shockingly irresponsible and somewhat dangerous. If revealing US misdeeds damaged our national interests, that's our problem, not his, and we obviously need to clean up our act. However, there is an awful lot of sensitive information which the government is quite right to keep secret, not because it hides evidence of their perfidy, but because leaking it simply creates messes. Stuff like which foreign nationals are (legally) cooperating with us, which foreign officials are problematic to deal with, what the political situation in a country is like, etc. I'm not convinced that it actually did as much harm as some have suggested - if people really did get killed as a result of the leaks, I'm sure the prosecution would have made a big deal about it - but we simply can't afford to let this kind of irresponsibility go entirely unpunished. Time served, a criminal record, and a dishonorable discharge seem like enough to me, however.
(On the other hand, from what I've read about Edward Snowden, I'd have a difficult time defending his prosecution under any circumstances, although I'm not very impressed that he sought refuge with the PRC and Putin.)