Yes, power corrupts, and the US government is exceptionally powerful and exceptionally corrupt. The NSA is included in this, of course.

We already know the NSA's power has been extensively abused with "SEXINT" (stealing people's private photos for personal enjoyment and likely blackmail), JTRIG (discrediting people not accused or suspected of any crime), PRISM (monitoring every communication via to/from data and other private data), not to mention others.

None of these things relate to foreign espionage or generalized signals intelligence. Snowden said it himself: the NSA is only about SIGINT in their press releases. Really, their purpose is social control and consolidation of power. Their spying does not stop at members of the public or foreign publics; whistleblower Russ Tice said long before Snowden that the NSA had a tap on Barack Obama as early as 2004.