Two things can be true. Snowden likely committed acts of treason for which the govt will pursue him. Snowden did pose a threat to US national security interests. If you look at the Schneier essay, he shared privileged info with journalists with poor operational security experience, one of whom crossed borders with a USB thumb drive full of docs. As a whistleblower, he might have enjoyed some legal protections if he had stayed within the lines, which would've at last implied staying in the US and dealing with the possibility of being tried. By fleeing abroad (to the PRC no less, then Russia), he made it easy to write him off as a traitor.
At the same time, he exposed behaviors that were clearly wrong in both government and industry, and spurred a number of people into actions that would not have been undertaken otherwise. For this he should be thanked. That doesn't erase everything.
It does erase it. If you (Snowden in this case) take an unlawful action out of reasonable necessity to defend yourself and others from the criminal acts of another (the US gov in this case), the latter is responsible for it, not the former (subject to certain limits like murder https://en.m.wikipedia.org/wiki/R_v_Dudley_and_Stephens).
It's pretty debatable whether the entirety of Snowden's actions can be attributed to "reasonable necessity to defend from criminal acts". He could, and did, encrypt his own comms, by which point it looks like he was pretty shielded. Dropping a lode of secret docs with journalists that seemed to have barely a grasp of security, that doesn't seem to have been the only reasonable action strictly to protect himself or even others against govt action.
He was under great personal pressure as a result of the actions of the original criminal (US gov pursuing him and canceling passport etc), and had very limited time and resources of his own.
He could have dumped it all on the net, but instead handed it to big name highly resourced journalists, who were definitely aware of the seriousness of what they'd received.
There's two ways people try to discredit Snowden. The first is the government absolutist way, that no-one can wrong the government even to remedy its own far superior wrongs. The second is the backdoor version of the same: to say yes we get why he did it, but to impose a burden of performance that only a superman could attain in the reality of that situation, falling short of which: "he caused more harm than he prevented".
He wasn't under strong direct pressure from govt until after fleeing with secret docs. The actions he took after that can't justify his theft and flight abroad. The theft and flight abroad, however, were disproportionate relative to the harm that he was suffering from personally (metadata collection, eavesdropping, both of which he had the technical knowledge to protect himself from).
I'm not trying to balance actions against one another in some form of equation to see what the net good is. That doesn't make sense. It reminds me of Dave Chappelle's skit about trying to process rape allegations against Bill Crosby. He was a great comedian. And probably a rapist. Well, Snowden is similarly complicated.
Comparing Snowden's actions with Cosby is disingenuous at best, sorry. Cosby drugged and raped dozens of women. All of Snowden's actions in their entirety were done with positive intent, regardless of any supposed missteps. Completely incomparable.
Not sure those documents had any legal right to be secret in the first place. Criminals (and that's what the people that broke wiretap laws became) should not enjoy the benefits of secrecy.
Snowden saw what happened to previous whistleblowers like Binney who did stay within the lines, and who achieved little or nothing beyond making themselves pariahs in the intelligence community.
He saw that he had to take a different path, even though it was one that was illegal. And I respect and honor him for being willing to make that choice.
It's a miracle (owed largely to the incompetence of "The Plumbers") that Daniel Ellsberg (RIP) both avoided prosecution and being murdered.
Heck, Assange never stole anything, his crimes were journalism and publication.
Also, 10 years on, there hasn't been any direct evidence of any "damage" from the revelations and it's doubtful the document archive is of any present-day risk to anyone.
At the same time, he exposed behaviors that were clearly wrong in both government and industry, and spurred a number of people into actions that would not have been undertaken otherwise. For this he should be thanked. That doesn't erase everything.