This is a tough call. I'm not 100% sure I buy the whole "he's a hero" mindset, since he potentially weakened his country.
To draw a parallel, what if somebody in the 1960s gave away some US secrets to USSR, just because they believe communism was a better regime. Does that excuse them, since essentially they were operating in the interests of the country? Or maybe not, because only 2% of Americans would agree with him. While 30% agree with what Manning did?
Furthermore, who gets to decide whether it's in the country's best interest? As seen from this thread, there are people with different opinions. I believe this is the reason why we have laws in this country. Yes, it can be a burden sometimes (for example, police cannot search suspects house without a warrant) and yes sometimes that could mean the criminal gets away - but Pfc. Manning broke one of our laws, and now he must pay.
And whether that was a worthy sacrifice, will only be known by him, and our future generations.
You know what's clean? There's a memo showing the President saying "our secret plan to destroy the United States just because it's fun is in place! Burn this document after my aide, who has promised to keep it secret, hands it to you!" And then the aide leaks the document to the Congress, and the nation is saved. Not much controversy - well, some like John of Sheffield who say the leaker betrayed his duty. But not most.
You know what never happens? That clean scenario.
Fact is, the US government lied to the US people, big lies, for decades, across presidents, on Vietnam.
When Ellsberg considered leaking evidence of that, oen fact was, doing so could 'provide aid and comfort to our enemy'. It might harm the 'war cause', it might get people killed as it affected the war. So, should he have just done nothing and kept silent, and let the lies continue?
Hardly.
Fact is, it's not that simple - but it's far, far easier to err on the side of NOT leaking, and saying "well, if thousands are killed by wrongdoing, a least the person didn't leak."
If ONE person is killed by an excessive, that seems far more harmful than thousands killed by protecting the system of our elected (or other countries' not so elected) officials. This is natural. Do YOU want to be the guy who gets a group somewhere killed? Of course not, so just play along and keep the wrongs secret.
So am I saying, be reckless and do the opposite just leak in case some good comes of it? Not at all.
What I am saying is that the price of NOT leaking when there is clear reason to do so can be very high - you might be part of a machine doing great wrong.
Daniel Ellsberg's position was, he morally could not be part of that, and keep the secrets he knew about the lies. He was disappointed that Congress was too scared to act on the information - and so he went to the press. He was worried, and was proven right to worry, that even if the public were informed of the lies, they might not do much about it. But wasn't the nation SERVED by having the fact its leaders had lied to the people so much on war exposed? So that it better put a stop to a wrong war as well?
When you go looking for the 'clean' scenario to justify a leak, you won't find it.
But even if it's found Manning leaked for spitefulness and not a noble reason, falling far short of your standards - look at the overturning of one or more dictators as a result, and recognize the simple fact that these things are complex, without simple answers, and that if you take the simple answer against the leak, the dictators stays in power.
Opposing or supporting leaks simply, both are very harmful. The fact is - you don't get clean answers.
Don't just ask, 'what if a leak caused harm?" Also ask, "how do we prevent overclassification leading to the strengthening of corrupt organizations?"
How do YOU bring down a Tunisian government and have democracy possibly take hold, if not with this leak? You don't. So do you just say, 'corrupt power is ok'?
It's not easy weighing. But as Mencken said, 'For every problem, there is a solution, neat, simple, and wrong.' That's the simple 'leaks are bad or good' solutions.