Within our lifetimes, this may well be a "crazy guy in a garage" project.
And the memeset I'd personally put my money on being the proximal cause is "humans are a blight upon Mother Gaia and should be removed at all costs". We don't have to imagine the existence of people who would push the Kill All Humans button if given a chance... we need merely read the right bits of the Internet. Sure, most of them would at least think twice... but there's enough that wouldn't.
And the memeset I'd personally put my money on being the proximal cause is "humans are a blight upon Mother Gaia and should be removed at all costs". We don't have to imagine the existence of people who would push the Kill All Humans button if given a chance... we need merely read the right bits of the Internet. Sure, most of them would at least think twice... but there's enough that wouldn't.