I want to preface this message with a disclaimer: I am in no way
   defending the actions of the NSA. I am merely attempting analysis of
   their motivations and their MO.
   [1]https://www.youtube.com/watch?v=H33r2weM6Zc
   Joel Brenner says,
   "There is in all the intelligence agencies a tension between doing
   offense better and doing defense better" ...and, "but unless you think
   0days are finite"
   Ok let's talk about this issue for a moment.
   - In the CTF world, it has been widely accepted that given a red team
   and a blue team of equal abilities, the red team always wins.
   - Academics say there is no way to create provably secure software.
   - Developers have a phrase, "there is no such thing as bug free
   software."
   - CISOs are using the term "risk" to describe pen test findings: a
   recognition that pen tests have become a measure of the risk that
   someone will find an exploitable flaw.
   Suppose that the NSA gave up on securing software because they view it
   as impossible. In fact, we know they view it as impossible because they
   have called it a Sisyphean Task.
   If there are diminishing returns from reporting software
   vulnerabilities to the vendor, then doing so is a losing battle. I hear
   people say that the NSA undermines the security of software by not
   releasing the vulnerabilities, but we know that historically companies
   have been very bad at actually fixing the vulnerabilities they are
   given. In some cases, a new product is released before a vulnerability
   is even looked into, thus rendering the effort useless.
   So, is defense dead? Is that an accepted fact these days? If offense is
   a type of defense, is the NSA perhaps aiming to use 0day for their
   offensive capabilities to effect a kind of defense? How would they
   accomplish this?
   Have any of you been to the Berlin Unterwelten? It is a tour of
   revision after revision of nuclear bomb shelters that could never
   possibly save the population they are tasked with saving. We are living
   in an age where there is an entire set of strategies that deal with war
   in a world with weapons so strong that no walls can possibly defend
   against them.
   Although the reach of nuclear weapons historically has been further
   reaching than so-called "cyber" weapons, that is changing. Despite the
   very many warnings from the infosec industry, that is changing.
   (Sometimes I think my Home Invasion 2.0 talk fell on deaf ears because
   smart home appliances are proliferating.) And in the future of smart
   homes, smart cities, even smart bodies, when everything is internet
   connected: everyone is vulnerable. Imagine cities that can be invaded
   without physical armies.
   If you were the NSA, and you believed these things to be true, what
   would you be doing?
   -Jen

References

   1. https://www.youtube.com/watch?v=H33r2weM6Zc