Mr. Snowden violated the terms of the NDA he signed when granted a security clearance. Instead of raising his concerns to appropriate internal sources, he admittedly leaked classified information to a foreign source.
If not prosecuted, it sets a dangerous precedent - essentially stating that it is acceptable to unilaterally decide what you as a clearance holder believe to be right or wrong, and that releasing classified information is not a crime.
The fact that he, as a very low level contractor, had no real understanding of the programs he believed to be "illegal" is not relevant, especially since all 3 branches of the federal govt were aware and deemed the programs legal and necessary. He is not a whistleblower. He is a criminal who has unquestionably made this country less safe.