The idea is that instead of packing a truck with explosives (which create a large but very localised detonation) you just deploy a swarm of these and let them take out whatever group of people you want to target.

Unlike in Black Mirror's Metalhead this isn't a scenario that requires long-lived machines. You just need them to be readily available at scale and to use facial recognition (or whatever -- the US already targets drone strikes based on cell phone signals I hear) to determine their targets.

Or hey, imagine a white supremacist group (not like the US has any of those) using them to just kill people of color. Terrorism doesn't need anything approximating a 100% success rate to achieve its goal.