NSA or any other agency won't gain anything by destroying Bitcoin because it's not a threat to the national security of any country right now and I don't think it will be any time soon. Plus Bitcoin is open-source right? So any tampering with the source code or placing backdoors would be easily noticed wouldn't it?
You really think, that a currency, that is openly competing against the USD and the banking system (and any other fiat) is not a thread? Not even after the USG has shutdown the Paypal account of Wikileaks and they started to accept Bitcoin afterwards? Not even after the SilkRoad case?
Check this:
https://bitcointalk.org/index.php?topic=1462981.msg14768412#msg14768412