One little mishap that happened in the past would hardly stop the Feds from pursuing the technology
There might be deeply rooted moral objections, though. I mean, it makes no sense that neither Hitler nor Churchill used massive amounts of poison gas in the aerial bombing of civilian centers of population in WWII. The aim was to terrorize: poisoning would have been an excellent means, and might have been considered the most humane approach imaginable as it would have stood the best chance of shortening the war and minimizing casualties. But poison gas, while stockpiled, was never used that way. And biological weapons were held back in even more curious circumstances: the various long-shot ideas of the Japanese on turning the tide of the Pacific War would have benefited from a terror warhead, but none of the exotic delivery systems, real or projected (balloons, submarine-based aircraft, infiltrators in minisubs or boats), ever received this sort of payload, apparently even on paper.
If the great villains of mankind shirk away from potent weapons in times of desperation, we can hardly assume the Feds would calmly wield the possible futuristic equivalents just because humans are about to go extinct.