Kant was a deontologist, meaning he firmly believed that an action should be determined as moral or immoral based on the motive behind the action (as opposed to consequentialists, who judge an action’s morality based on its consequences). According to Kant, since we have the ability to deliberate and provide reasons for an action, moral judgment should be placed on those reasons why an action was performed. While it is important that our actions have good consequences and we should always try for that result, consequences are not affected by reason, and therefore, reason is not completely responsible for the consequences of a particular action that was endorsed by reason.
Reason can only be held accountable for endorsing a particular action. Therefore, we can only judge motives and actions as being moral or immoral. Since morality is determined from reason, this means that goodness and badness also stem from reason.
Kant claims that acting badly is violating the maxims created from one’s own personal reason, or creating maxims that cannot consistently be viewed as universal laws. In other words, badness is the result of laws of reason being violated. From this notion, we can conclude that immorality is actually a type of irrationality because the laws of reason are being violated. By acting immorally, Kant believes that we become less rational human beings, thus weakening our humanity. We can only stop ourselves from doing things against our better judgment by behaving rationally.