This is public knowledge but never spoken about, Snowden was looking for years [0] to find something to leak. He didn't stumble across this and have an overwhelming moral urge to do "the right thing."
I'll take the very unpopular opinion: I have a very strong feeling he is almost completely wrong in what he thinks he knows. [1]
[0] If you read the timeline you will see this. I'm looking for better sources, as I read it directly, but can't find at the moment. http://abcnews.go.com/Blotter/timeline-edward-snowdens-life/...
[1] This tone of feeling wronged and spied upon is borne of complete ignorance. http://news.yahoo.com/edward-snowden-completely-wrong-104432...