Monsanto an American multinational agrochemical and agricultural biotechnology corporation.
Monsanto has frequently been criticized due to its products and policies.
General criticisms of "synthetic" and "genetically changed" substances
One cause of criticisms of Monsanto is not due to something specific done by Monsanto but is due to general fears of "synthetic" substances and especially due to general fears of "genetically changed" substances and "genetically modified foods".
This is dubious since plants and animals produces numerous more or less poisonous substances (for example, many plants produce poisons in order to not be eaten) and many synthetic substances are beneficial (for example, many medicines).
Humans have been changing the genetics of their domesticated plants and animals for thousands of years through methods such as selective breeding
Critics of "genetically changed" substances by more recent methods such as genetic engineering may use dishonest arguments such as pointing out that some such substances may be harmful and then misleading imply that therefore all such substances are harmful.
Such critics may themselves have concealed economic motives by being more or less covertly associated with the large "organic farming" business. "Organic farming" has a number of problems such as prohibiting substances solely due to being "synthetic" (rather than due to evidence of actual harmfulness), low productivity, and various other issues.
Criticisms of specific products and policies
Another form of criticism is of various specific Monsanto products and policies (past and present) which have been argued to be harmful.
Monsanto and "genetically modified foods" more generally may also form part of various conspiracy theories with "genetically modified foods" claimed to be intended to cause deliberately negative effects.
Another view is that such conspiracy theories may have the subtle effects of increasing opposition to eugenics and movements associated with eugenics.