He apparently compromised some legitimate national security intelligence but I don't know how much. I briefly searched the Guardian website but didn't see any documents with classification markings on them.
To me it seems pretty arrogant for one person to unilaterally decide to potentially jeopardize the lives of millions by illegally publicizing the work of thousands, many no doubt seriously committed to the actual intelligence mission and not just greedy profiteers. If there aren't already, there need to be channels available to both NSA employees and contractors to register their complaints and expect reasonable action without fear of retaliation.
Maybe he feared losing his job and wanted to "cash in" while he still could. 
Maybe he was a disgruntled employee who wanted to get even with a boss who nailed him for something. Maybe he's just a "nobody" who wants to be "somebody."
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