Assume a distant future where all humans have nanobots in their bloodstream that keep them alive indefinitely.
Now let's say there is a way to override those nanobots so that they can instantly be triggered to kill their host. What would be the quickest way for the nanobots to kill their host? Would it be to attack the heart, the brain, or all organs at once? Is there another quicker way?
For the sake of my story, it would be best if the death looked peaceful to the outside observer, but it doesn't necessarily have to be painless. Although, bonus points for anyone who can also give ideas for how the nanobots could kill someone in the most painless way possible.