4. Agent Orange
The United States Army created Agent Orange, an herbicide and defoliant, back in the 1940s. To prevent rebels from using cover in densely forested areas, the substance was intended to be sprayed on shrubs and trees. But when the US deployed it in Vietnam in the 1960s, it became iconic.
As expected, Agent Orange showed some ugly adverse reactions, even for the soldiers who used it against the Vietnamese. In addition, this herbicide causes birth defects and other serious health issues.
5. Dynamite
Although Alfred Nobel is undoubtedly one of the most significant figures in history, his creation of dynamite, while well-intentioned, had tragic results. Originally designed for mining because it could be used for rock blasting and excavation, dynamite has since been used to blow things up for bad purposes, including people.