They are already flamethrower drones (as seen on Grand Tour, in one of the funniest bits of television ever).

I think as far as availability and getting away with go, you're probably better off cropdusting people at your venue of choice with some Norovirus. They wouldn't all die, but many would get painfully sick a couple days later and they'd spread it to many others because they feel healthy and fine at first.

The Shittening of 2019, they'd call it. Toilets wouldn't know what hit them.