The statement "The human being is evil" cannot mean anything else than that he is conscious of the moral law and yet has incorporated into his maxim the (occasional) deviation from it. (6:32)
In this brief passage, Kant introduces two ideas that animate all of his thinking about religion and morality. First, he says that human beings are evil when they intentionally decide to act against their better judgment. Second, he says that immoral action is not simply a matter of acting on immoral impulses. Humans act according to maxims, or principles of action, and humans are evil when they knowingly create a maxim that allows occasional bad behavior. Evil involves intentionally formulating a principle that sanctions illicit actions. Evil, therefore, is not simply an action, it is a premeditated action performed in accordance with a set of principles.