The widely accepted definition of 'national security' means the survival of a state. To say that Snowden endangered national security is to say that he took actions which could credibly precipitate the end of the purportedly democratic regime which is in force, and, for example, lead to an anarchy, dictatorship or a return to British rule.
I personally don't believe this to be true; if anything, he may have enhanced national (and international) security against a future shift to a higher and unpopular level of authoritarianism.
Certainly if the state has secret powers they have the power of surprise over some criminals and can prevent some crimes they might not otherwise, but it is folly to optimise for one thing (low crime) to the detriment of all others (personal privacy, governance by consent of the people, rule of law, balance of power of entrenched interests, due process rather than being targeted on secret evidence), and if the government is doing that, the public have at least a moral right to know.