The first two subsections of Part One introduce the idea that human beings are inherently evil. It is not yet clear why people naturally tend to engage in evil or immoral behavior. At this point, Kant explains in greater detail what he means when he says that human beings are inherently evil. He also tries to explain why people tend to do bad things, instead of acting morally.
On the first score, Kant simply emphasizes that human beings are inherently evil because insofar as people act on anything but duty, they act against morality. This explanation highlights the difference between Kant's definition of moral evil and the common definition to which most people subscribe. Most of us think that an action is evil when it is maliciously performed, almost impossible to understand in its cruelty, and hurtful in its consequences. Kant emphasizes that people's intentions, even aside from the outcome of those intentions, are evil.
The second question asks why we have a propensity for evil. Kant explains that when he says humans are evil by nature, he is not making an a priori claim. An a priori claim is one we can know to be true or false without consulting experience or science. Kant says that his claim about human nature can and must be proven by examining experience.
Kant contends that we must be free to engage in evil behavior, for if we did not freely engage in immoral behavior, it would make no sense to say that we are morally evil. The problem of evil has to do with the relationship between our ability to freely choose evil or good, and the demands of morality. Kant also explains that we are inherently evil because we use our free will to subordinate the moral law to our own whims and desires. We cannot ever completely reject the moral law, according to Kant. The moral law is part of what makes us rational creatures. But we certainly can demote the moral law, and the tendency to do this is what makes us essentially evil.
Kant claims that we know people to be inherently evil because our experience with others proves it. This is not a philosophical claim, but an invitation to look around and confirm his thesis by noticing the evil and immoral behavior that surrounds us. If we do not see a great deal of immoral behavior, perhaps it is our powers of perception that are to blame.
Kant says that human beings are inherently evil because they consistently put their own interests ahead of the moral law. If we pass off some of Kant's absolute statement of people's evil as rhetorical exaggeration, we can decipher that in his world view, people genuinely care about morality, but typically face situations in which their interests have to come first. Kant still does not explain precisely why human beings have a tendency to take the immoral path, beyond repeating the idea that humans do evil things because they act against their sense of duty. He says that we ignore the moral law and duty without saying why we do this. Because Kant does not posit a theory of why humans do evil, he risks leading the reader to think that people do not choose to do evil, but do evil because it is beyond their control to choose duty.