I hate Monsanto more than the vermin and weeds they poison. But your statement just isn't true. There are differences in organisms' metabolisms, and it's perfectly possible for one substance to be deadly to one (class of) species, yet harmless or even nourishing to others.

Take antibiotics, which kill bacteria but have mild-to-none effects on humans (and most of the negative side effects on humans stem from their killing of even beneficial bacteria).