Even neutron bombs are fairly blunt weapons and will do damage and leave lingering radiation. A massive swarm of drones won’t do as well with people sheltered though for complete depopulation unless they can IR/Radar know where people are and just blow up with a big payload. However, that’ll also do a lot of physical damage.
Perhaps this is why there are numerous reports of ultra-sophisticated laser weaponry on the USA assembly line.
With IR, they could just land on high points and watch for anything warm to move. Then they pop up, shoot one bullet into it and wait again. One drone could kill 100 people over several weeks until its fuel runs out and then self-destruct. That is how I’d make them.