So, yesterday it was nuclear Armageddon that was the ultimate crisis, but today it's terrorism? I can't help but be amused by Sawyer's reply.
Today's terrorism is no threat to human existence because WMD's are for the most part difficult to produce and require very specialized equipment to produce (any halfway decent nuclear physics student knows how to put together a nuclear bomb but he will lack essential materials, i.e. weapons grade plutonium for example).
However the danger is in the knowledge itself so what happens when this red line can be crossed and someone is able to put together a method of mass destruction with readily available equipment? I believe that's what he's talking about.
Now as to where the aliens are.. on a philosophical standpoint it might be that we are simply not that interesting to begin with. We are volatile, fearful of new things, greedy and vengeful and the simple revelation that there are Aliens might topple our entire worldwide system because of mass panic.
In the event aliens understand us and have been observing us what do they have to gain by destroying our civilization? They might just leave us alone for a few decades or centuries and then check back if we're still around and how we do and otherwise concentrate on other projects.
It sounds dumb and overly simplistic, kind of their version of the Prime Directive but it could be that simple. Any alien civilization that's hostile could have easily attacked us by now with little risk.. the few things we have that could do damage to something in orbit might not be nearly enough to take out a dedicated invasion fleet. Heck, them taking out our satellites would already cripple us because we've become so dependent on them.
So if there was contact or if aliens exist and have discovered us i assume they are either benevolent or simply don't care about us.