Imagine a handyman hired to renovate a house.
During the projects that spread over days, every time the owner was out of the house, the handyman rifled through the owner's closets and desk, go through his computer, and found out the owner was stealing from his employer, embezzled his clients and even from the local hospital. The handyman then decide to expose the man to the police.
Would anyone hire this handyman later ? Not likely. Secrets by themselves do not harm anyone and honest people have their secrets that they guard as fiercely as crooks and governments guards their secrets. So once it comes out that this handyman was able to expose a criminal among the people, everyone will know that this was possible only if the handyman committed at least an immoral, if not illegal act, and that while the people are glad that a criminal is out of their immediate neighborhood, they would not want a criminal working in their homes, no matter how much they may applaud the handyman in public for what he did.
The point is that Edward Snowden knew exactly what he did was beyond immoral but criminal.
Snowden was NOT an NSA employee. He worked for the NSA via Booz Allen Hamilton, a contractor of various services to the government. Snowden was a technical specialist: an administrator of various computer systems. Snowden did not perform any 'NSA stuff'. Rather, as a contractor, he maintains the computer hardware so that NSA employees can do their 'NSA stuff', the activities that gather strategic intelligence for US political leadership. At best, Snowden probably had better than average knowledge and educated guesses on those 'NSA stuff' than the average American. As a computer systems administrator, Snowden had high permission level to access deep layers of computer systems and data storage systems in order to do his daily duties efficiently, but that does not mean he was any kind of intelligence gatherer, analyst, or decision maker.
This is why Snowden had to deceive those who do perform top secret 'NSA stuff' to allow him access to their data. This fact about Snowden is usually glossed over by mainstream news and unknown to those who would beatify Snowden, not because they have any respect for the US Constitution, as if they have any serious knowledge and understanding of the US Constitution in the first place, but for spite of the US.
This is why Snowden had to hide behind the whistleblower label and the public distaste for the what and the how the NSA had gone too far, the same way the nosy handyman had to hide behind the atrocious crimes of the house owner to distract attention to his own violation of trust, a trust that is expected and always manifest between client and provider of services. Snowden probably knew that what he did was criminal enough to put him in prison despite any good that may arise out of the NSA exposure and possibly that understanding came from legal consultants. What a story that would be, that Snowden consulted a legal team and they told him -- no dice.
Past NSA whistleblowers, to the best of public information, did not deceive their coworkers to gain information. They observed and record what they personally saw and experienced, then they followed the chain of command like they were supposed to. Yes, their lives deteriorated because the organization went after them, as expected of any large organization with a reputation to protect and a sense of self preservation, but at least those whistleblowers can live with the honor that they did not take national secrets to the enemies.
Snowden does not need to deceive NSA employees to expose the 'NSA stuff' that so offends everyone after the fact. Neither Russia nor China need to serve as conduit for Snowden's slow releases of those 'NSA stuff'. The New York Times or The Atlantic or any major newspaper will gladly be his mouthpiece.
This is why unlike past NSA whistleblowers, like Thomas Drake, there is a fierce divide among legal experts and moralists on whether Snowden is a genuine whistleblower who did the country good, or an egotistical fool too deluded to recognize his treason.
