A bio weapon wouldn't work because the faster and deadlier a bio-agent is the more likely it is to burn-out before infecting and killing the entire West.
A chem weapon would be impossible to disseminate in a way that would kill everyone in the West.
Nuke explosions make huge areas uninhabitable and they destroy valuable infrastructure. Nuke poisons are tough to spread over an area wide enough in amounts big enough to be lethal.
The ONLY way enemies of the West could hope to destroy the West is by CULTURAL attack - and this is precisely what the postmodern left and the "creeping sharia" jihadists are doing.
Patiently undermining our economy, our national defense, our ability to exploit the natural bounty around us, undermining ancient institutions and and values - like marriage, and inculcating a desire to be childless. And undermining our will to defend our civilization.
And so we may very well disappear with a whimper and not a bang.
WMD: a whimper of mass destruction.