The title of this blog post is intentionally provocative, but I stand by the general idea, as I will make it clear throughout this post.
Kant developed a particular approach to reconciling the rationalist and empiricist philosophies of previous generations which is called transcendental idealism. On the one hand, the rationalists were metaphysical realists who understood ordinary objects of experience in categories that strictly speaking were not empirically discovered. The empiricists, on the other hand, decided to limit their claim to knowledge to what can be directly observed through the senses. David Hume and his extreme metaphysical skepticism represents the logical terminus of empiricist epistemology: even causality, as basic a concept as it is to understanding the real world, became instead a habit of the mind imposed on objects, with no real (i.e. extra-mental) existence.
Kant sees the importance of the metaphysical concepts of time, causality, etc. for understanding the world, yet he grants the empiricist claim that these concepts are not gained through sense experience. His resolution is to call these concepts "transcendental": they are preconditions of experience, without which intelligible experience isn't possible. These act like a "filter" through which the mind understands the world. In a sense, these concepts are imposed upon the object of experience to make it intelligible. The result of this is that genuine metaphysical knowledge of reality in itself is impossible; we can only know things as they appear to us.
Now this means that we cannot ground ethics in metaphysics. We cannot construct an ethical system on such a notion as human nature and natural teleology, because we cannot know such things. Kant cannot claim to know what human nature consists in, because this is not knowable, given his distinction between the noumenal and phenomenal, that is, between the thing in itself and the thing as it appears to us.
If metaphysical knowledge is impossible, then ethics quickly becomes relativist and subjectivist. That is because there is nothing in the real world by appeal to which you can justify or motivate your actions; the "real" world is cut off from your access. Instead, ethics can only be grounded in the contingent and subjective impulses with which you find yourself.
What happens when your impulses are not good? What happens when your impulse is to engage in sexual activity with someone other than your husband or wife, or when your impulse is to manipulate another person for your own monetary gain, or when your impulse is to dominate the other person out of a fear of the exotic, or whatever? There is now nothing beyond your own impulse and desire to which you can appeal to justify or condemn your own actions. Very quickly, then, given the fact of human depravity, this kind of metaphysical skepticism will translate into debauchery and sin. Lacking a principled reason to prefer conscience, and lacking a principled reason to prefer self-control, you might quickly fall into a kind of crass hedonism that is utterly repulsive.
Of course, you might say that you have a conscience which condemns you when you do those things. But you cannot provide a principled reason for preferring conscience to the impulse for evil which has arisen in you; there is no metaphysical basis for preferring one to the other. More than that, conscience is not an infallible guide to right and wrong: some persons consciences are seared, and you have no grounds for convincing them that they are wrong, nor even of justifying the judgment that their consciences are seared (i.e., defective or dead) apart from your own baseless moral intuition.
It is not difficult to see that this kind of thinking can motivate a kind of totalitarian and intolerant attitude toward differing moral opinions, too. Reasoning with others on moral matters by appeal to objective criteria is no longer possible, because we can have no metaphysical knowledge. The only way forward is empty appeals to intuition or else shouting the other person down, or worse.
So a skepticism about metaphysics can quickly or slowly make you a bad person.