Killer self-replicating nanobots are theoretically possible, therefore they're inevitable. This is undesirable, but what's to stop someone from making 1? (1 is all it would takes)
They could be programmed to enter every person, go into their heart, replicate there until there's enough of them, then at a specified time coagulate together in the aortic valve shutting off bloodflow and killing every person at once. Or maybe 1 specific race or religion of person. Depending on the creator's intention.