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?
The NSA works for the interest of the USA. Which means theoretically the USA would need the control over their money because controlling it gives power over the economy.
If someone would take this out of their hands then be sure that they would fight really hard.