Well, one would think that that those that hired Snowdon had all the resources of the federal government to do a complete background check and assessment of the people they hire.
I think they did, and Snowdon is most likely much smarter and perhaps more crafty, than he is given credit for. After all, he is still alive. I am not saying that I agree with what he did, but I am conflicted about him on the issue in regards to his disclosure of how the government has violated the Constitution in regards to American citizen surveillance. At least we know for sure now.
As Theoden says in The Lord of The Rings [not the hideous movie, the great book] "Oft evil will shall evil mar."
Just because Snowden accidentally may have done some good, that doesn't make him a good person.