Had Snowden stopped there, he might have had a case that what he did - while illegal - flowed from decent motives.
Unfortunately, he didn't stop there. Instead he downloaded several laptops worth of our most closely guarded secrets, and took them to first China and then Russia. He didn't just release the Metadata program, he also released how we tracked attack plotting from al-Qa'ida and the Taliban. He released how we collected against other government intentions, like, for example, the Russians. Where I work, we have several people who had to be.... hurriedly removed from overseas places as soon as we realized that he had given their names up to the people who they were collecting against. They are forbidden to travel to significant chunks of the globe for the rest of their life, because if they go there, they'll get picked up. The NSA was hugely set back by the destruction that Snowden left in his wake - when you burn a SIGINT collection capability, you generally burn the entire program.
Snowden isn't a traitor primarily because he released the Metadata. He's a traitor because he helped the Taliban improve their ability to kill US Servicemembers overseas and then became an agent of a foreign nation with the intent of doing as much damage to US foreign policy as possible.
