A cannon shell from a battleship can kill hundreds of people or just one. Depends on whether you're shelling a crowded town and hitting a skyscraper, bringing it down - or a lone house on a beach where only one person lives.
I wouldn't call it "engineered", that sounds like Sarin was somehow altered to provide this property. It's rather that the final production step is so reactive that you can use it to produce Sarin on the fly (thus making it a binary agent). But that's nitpicking.