If a private company hired you to kill people...
... and you broke their NDA...
... they will hire someone to kill you.
I get what you're trying to ask, but oh boy have you picked a silly example.
Long story short: Uh, basically you have to sue or be sued, to find out how enforceable an NDA actually is or isnt.
But:
Big company have money.
You? Not have as much money.
NDAs are mostly effective tools of stifling speech by way of intimidation, not by way of legal legitimacy.
Sure, I'm sure many scenarios exist where a clause of an NDA is enforceable if you violate it... but there are so many different possible laws and jurisdictions that come into play... you'd have to be a lot more specific.