Consider the mixing requirements of carfentanil: one insufficiently ground grain of the chemical is lethal. And the people making the end-product are amateurs.
One kilo from China was confiscated in Canada. Certainly a bit hyperbolic, but "According to the Canada Border Services Agency, the shipment contained 50 million lethal doses of the drug, more than enough carfentanil to wipe out the entire population of the country[...]". Divide that by ten and it's still terrifying. Dump a small bag over NYC Times Square or Mumbai and you'd create mass casualties.
This isn't some crazy, Bethesda-managed super-bug or insecticide. It's a bog standard, synthetic opioid.