DDT
Chemical used initially during World War II to kill disease-carrying lice and mosquitoes, and, in following decades, sprayed all over the planet to prove our superiority over our insect co-inhabitants; had the pesky side effect of causing cancer, but its main hobby was killing off entire populations of species and thus disturbing the delicate balance of the ecosystem. Usage largely curbed, thanks to the 1995 Stockholm Convention-an event which also unfortunately coincided with an uptick of malaria cases.