His oath was to "defend the Constitution against all enemies, foreign and domestic."

The NSA was (and is) violating the Constitution, and what he released exposed that. That makes the NSA an enemy of the Constitution, and this means that Snowden was fulfilling his oath.

Is he a criminal? Possibly. He broke the law, and violated the regulations of his employment.

Is he a traitor? No. He was fulfilling the conditions of his oath.

Daniel Ellsberg was in exactly the same position. History has vindicated what he did when he released the Pentagon Papers, but at the time the same charges were leveled against him. I saw him in a talk not long after the Snowden revelations, and these are the points that Ellsberg made.