"Micro- and macroscopic, self-replicating robots that can invisbily build or destroy anything. Vast sums of money are being poured into nanotechnology.
Scientists have already created a nano-cyborg, by fusing a tiny silicone chip to a virus. The first thing they found out is these cyborgs can still operate for up to a month after the death of the host.
Some day there may be nanobots in your brain. They can form their own neural pathways, meaning they can use your brain to keep operating your limbs after you've deceased and, presumably, right up until you rot to pieces in mid-stride. The nanobots will be programmed to self-replicate, and the death of the host will mean the end of the nanobots. To preserve themselves, they'd need to transfer to a new host. Therefore, the last act of the nanobot zombie would be to bite a hole in a healthy victim, letting the nanobots stream in and set up camp in the new host. Once in, they can shut down the part of the brain that resists (the cortex) and leave the brain stem intact.
It should be more than clear by this point that the goal is to develop responsible researchers. Someday runaway microscopic nanobots may end civilization by flooding the planet with the cannabalistic undead."

"Woldn't this make a headshot ineffective, since the nanobots already know their assigned task?"