In theory, you can do this today, in your garage.
Buy a quad as a kit. (cheap)
Figure out how to arm it (the trivial part).
Grab yolo, tuned for people detection. Grab any of the off the shelf facial recognition libraries. You can mostly run this on phone hardware, and if you're stripping out the radios then possibly for days.
The shim you have to write: software to fly the drone into the person... and thats probably around somewhere out there as well.
The tech to build "Screamers" (see: https://en.wikipedia.org/wiki/Screamers_(1995_film) ) already exists, is open source and can be very low power (see: https://www.youtube.com/shorts/O_lz0b792ew ) --