You can find cheap disposable drones for £15 on Chinese sites. Weaponizing them should be pretty easy.
The only problem is that the novelty of the approach would be such, that it would bring too much visibility to the attack. Most bad actors don't want to be detected: "he fell from the window" or "his plane crashed" leaves the door open to coincidental causes. Drones are noticeable, particularly in swarms. They would be useful only in "showy" attacks, i.e. open terrorism, a tactic that is largely out of favor after 20+ years of substantial failure.