I’m no proponent of capital punishment, but to take the question seriously: as an extreme measure to disincentivize extreme externalities that would otherwise never be priced in. In the status quo, captains of industry have something close to a call option on the social utility of their products. They capture much of the upside (they get paid when people enjoy their goretex jackets), but are pretty free to socialize tail risks (relative to the harm, they have little to lose from poisoning thousands. Maybe a lawsuit. Maybe they have to sell the third house.) A society could decide that this is not a desirable incentive structure.

There’s a pirates-and-emperors aspect to this: such measures are taken in some societies to disincentivize O(1) violent crimes. Why (really, why? It’s worth thinking about!) are they rarely taken for O(n) crimes?