don't believe that contractors swear an oath to protect the Constitution, and even if they did, who did Snowden vet his personal interpretation of the Constitution with? Nobody, I expect. Assuming his intentions were actually "good," he just decided he didn't like it and broke the law. As to the constitutionality of the programs, Professor Geoffrey Stone of the University of Chicago Law School has some views on that [volokh.com].
I don't know about his duty to the constitution, but in my view he acted ethically, if not legally. He saw something that he thought was wrong, and harmful to others, and tried to stop it. To me this is a higher good than merely protecting the Constitution. I would hope that everyone would do the same, if in his shoes, or in like situations. This is especially true in situtations like this, where no one who matters (i.e. not the powers that be) are likely to be harmed.
As for embarrassment... Big whoop. If you don't want to be embarrassed, don't do anything embarrassing. If your action bite you in the ass eventually, that is your own fault, and you don't get my sympathy. Especially when it is a Government operating outside their purview (the will, and welfare of the American people). I don't give a shit if it is "my" government, or a "bad" government like China, the standards of judgement are the same. An act is evil, or good, no matter who commits it. I fail in nationalism, surely, but history has taught me that nationalism is rarely a good thing.
Snowden is likely an asset of Russian intelligence.
If there is any proof of this, then it is worth talking about. If not, it is base speculation and not really worth entertaining. Might as well say that Snowden is working for the greys, to take power from the lizard aliens who control the American government. Both statements are pretty much equivalent at this point, being somewhat meaningless conspiracy theories. That said, their might be more than meets the eye with the Russian connection, but until there is some information it is pointless to speculate. We're also dealing with intelligence communities, so whose to say that most of the information on this tract isn't psyops, or whatever?
I wouldn't rush to judgment that the Snowden leaks are a good thing. The West may come to regret them greatly. These revelations will be playing out in events over the course of years.
My government might regret them. I won't. Even if they are of dubious origin, if they are true, then we deserve the egg in our face. And further, those that permitted this state of affairs should face some consequences. Even if this isn't good for my country, or me personally, I'm find with whatever fallout there may be, since consequences are always deserved for bad behavior. This is probably a rare point of view, but I'll stick with it. Ethics trump power, at least in my book.