U.S. government medical researchers intentionally infected hundreds of people in Guatemala, including institutionalized mental patients, with gonorrhea and syphilis without their knowledge or permission more than 60 years ago.
Many of those infected were encouraged to pass the infection onto others as part of the study.About one third of those who were infected never got adequate treatment.
Of course, another reason is that irresponsible leaders with ulterior motives benefit from the US being perceived as a monster:
Venezuela VP falsely claims US behind coup attempt in Ecuador. Venezuela is playing politics rather than helping Ecuador's president.