Not all drones 'hover', or are quad-copters. Most drone-swarms have been demonstrated using drones that fly like airplanes. 20 years ago when I started building RC planes, the smallest RC planes were about 6 inches wide. Many people are working on drones today that are insect-sized.

A mosquito-sized drone carrying a neurotoxin certainly would be used to carry out assassinations, crowd control, etc... And certainly would be a scary prospect.