Final answer:
A bad reason to believe in ethical egoism is to suppose that common-sense egoism is the view that egoism is a vice.
Step-by-step explanation:
A bad reason to believe in ethical egoism is to suppose that common-sense egoism is the view that egoism is a vice. This view involves putting one's own concerns over those of others to an immoderate degree. However, ethical egoism goes beyond common-sense egoism by claiming that it is always right to aim at one's own greatest good, even if it means disregarding the interests of others.