I agree, what you're referring to is very nasty stuff, and yes, you could easily kill a lot of people with it. You'll still have trouble wiping out an area larger than a stadium with it, though. (It's certainly possible to do though, if you have enough resources. Maybe something involving crop dusting planes, I dunno.)

On the bright side, anyone plotting to use that particular agent to kill people with is probably going to end up killing themselves with it before they manage to get anyone else.

Just for fun: "A Tall Tail" by Charles Stross