I think one of the fundemental problems is people have this odd belief that physical authority somehow imparts moral authority.
While the requirements to become a policeman (for instance) include a modicum of intelligence and the lack of a criminal past, they obviously aren't insurmountable.
Likewise, the NSA and our intelligence agencies are staffed with people who A: like the idea of being spies and B: have passed whatever requirments are set up. But they're only people. There is nothing in particular that prevents them from being bored, superstitious, excessively curious or just plain stupid. And human nature itself suggests that the less supervision or accountability there is, the more likely for abuse to turn up.
The amount of covering up already going on suggests that bad behavior has already taken place but the sad fact is that if what's already taken place doesn't come to light, then the activity that eventually WILL cause the house of cards to tumble will probably be something more appalling than I have the imagination to even consider