If China wanted to indiscriminately kill that many, it would crop dust populated areas with drones or dump a few hundred pounds into water supplies. As it is, appears to just be targeting undesirables that they don’t want to deal with—when they take over?
But how hard would it be for a decently funded terrorist organization to get their hands on a few KGs? For that matter, the Cartels aren’t too far removed from a terrorist organization. I’m not real comfortable with these dirtbags having tons of this crap at their disposal. As far as China is concerned, they’re mostly just interested in eroding our society I think. They understand the deleterious effect that widespread opiate addiction has on a society. Read about the Opium Wars and the subsequent Chinese century of humiliation.