Here's just one scenario.
UPDATE: I've now deleted the scenario because I've spoken with a true authority who says it's feasible. I don't want to take the chance of encouraging terrorism.
An clearly unfeasible version of that I posted months ago has been reposted on one or more Internet sites but leaving that there presents insignificant risk.
I can, alas, think of other seemingly feasible solo-actor Armageddon plans. Among the world's seven billion people, I'd imagine that at least one deranged but brilliant person could develop and execute an Armageddon plan that would actually work.
I am a bit reassured by this: If such an act were so likely, why hasn't it happened yet? But only a bit.