Let's say your best friend swears you to secrecy before he will tell you something juicy. Then, after you swore to keep his secret, he tells you he raped and murdered a young girl. Who in their right mind would believe the promise to keep the secret was more important than an attempt at justice? Or to inform others of the danger he poses? Who would be angry with you for breaking your oath to the murderous rapist?
And why is the Edward Snowden case any different?