Comment by photochemsyn

This is essentially what the pesticide and herbicide industries have been doing since their inception, i.e. designing molecules that efficiently kill animals, insects and plants. It seemed like a miracle at first, but the long-term consequences of things like persistent chlorinated aromatics and their derivatives (Agent Orange and dioxin for example) eventually appeared in human populations.

The development of the toxic nerve agents (organo-phosphate compouds mostly) in particular was a side effect of research into insect toxins. The nerve agents were discovered in this manner, they worked too well. Nevertheless, these pesticides were deemed safer than the organochlorines because they degraded fairly rapidly after application (although they are implicated in nerve damage related diseases like Parkinson's in agricultural areas).

Insect infestations are indeed a big issue in agriculture and can wipe out entire crops if not dealt with, but there are plenty of options that don't require applications of highly toxic or persistent chemicals.

Otherwise, this is just another of the many issues modern technology has created. Smallpox is another one - in the late 1990s, there was a great debate over whether to destroy the last smallpox samples - and then in the mid 2000's, someone demonstrated you could recreate smallpox by ordering the appropriate DNA sequences online and assembling them in a host cell. Then there's the past ten years of CRISPR and gain-of-function research with pathogenic viruses, a very contentious topic indeed, and still unresolved.