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.