The Categorical Imperative is all about generalizing specific experiences into moral rules based on supposed laws of rationality that really don't exist. It's like if you had to act superrationally, all the time, and that's what Kant's view of morality is, and he says that it's logically contradictory to act otherwise, which leads to doing stupid things like not defecting against naive opponents in the PD. If everyone actually did abide by the Categorical Imperative, there would be no theft and no murder, but also no artists or scientists, only boring farmers with their individuality stripped to a bare minimum. It would be awful.

He applies a variation of the Worst Argument in the World to the definition of "should" and to morality itself, as well as all actions that you're considering the morality of, basically. So, for example, he argues that anything that can be considered theft is irrational and immoral because if everyone committed all acts of theft imaginable, society would collapse, and thus the idea of property wouldn't exist, and thus we would have created a "contradiction", and thus that would violate a universal definition of the word should, and it would thus be immoral (this isn't me being dumb and misapplying his ideas, as he actually makes this line of argument somewhere or other. It's a good example of why I find him idiotic).

Relevant passage from Nietzsche's The Gay Science (Unfortunately, this is somewhat longer than I remembered. However, it's a good read, and Nietzsche's sarcastic tone here is undoubtedly amusing. And, anyone who wants a serious discussion of deontology is just asking to be inundated with walls of text anyway, so they basically deserve it [this parenthetical is somewhat tongue in cheek, as well.]):

Long Live Physics!

How many men are there who know how to observe? And among the few who do know, how many observe themselves? "Everyone is furthest from himself" all those who try to harness the self know that to their cost - and the saying, "Know thyself," in the mouth of a God and spoken to man, is almost malicious. But that the case of self observation is so desperate, is attested best of all by the manner in which almost everybody talks of the nature of a moral action, that prompt, willing, convinced, loquacious manner, with its look, its smile, and its pleasing eagerness! Everyone seems inclined to say to you: "Why, my dear Sir, that is precisely my affair! You address yourself with your question to him who is authorised to answer, for I happen to be wiser with regard to this matter than in anything else. Therefore, when a man decides that this is right} when he accordingly concludes that it must therefore be done, and thereupon does what he has thus recognised as right and designated as necessary then the nature of his action is moral!" But, my friend, you are talking to me about three actions instead of one: your deciding, for instance, that "this is right," is also an action, could one not judge either morally or immorally? Why do you regard this, and just this, as right? "Because my conscience tells me so; conscience never speaks immorally, indeed it determines in the first place what shall be moral!" But why do you listen to the voice of your conscience? What gives you the right to think that such judgements are true and infallible? For this faith - is there not a further conscience for that? Do you know nothing of an intellectual conscience - a conscience behind your "conscience"?

Your decision, "this is right," has a previous history in your impulses, your likes and dislikes, your experiences and non experiences; "how has it originated?" you must ask, and after wards the further question: "what really impels me to give ear to it?" You can listen to its command like a brave soldier who hears the command of his officer. Or like a woman who loves him who commands. Or like a flatterer and coward, afraid of the commander. Or like a blockhead who follows because he has nothing to say to the contrary. In short, there are a hundred different ways that you can listen to your conscience. But that you hear this or that judgment as the voice of conscience, consequently, that you feel a thing to be right may have its cause in the fact that you have never thought about your nature, and have blindly accepted from your childhood what has been designated to you as right: or in the fact that hitherto bread and honours have fallen to your share with that which you call your duty, it is "right" to you, because it seems to be your "condition of existence" (that you, however, have a right to existence seems to you irrefutable!) The persistency of your moral judgment might still be just a proof of personal wretchedness or impersonality; your "moral force" might have its source in your obstinacy or in your incapacity to perceive new ideals! And to be brief: if you had thought more acutely, observed more accurately, and had learned more, you would no longer under all circumstances call this and that your "duty" and your "conscience": the knowledge how moral judgments have in general always originated would make you tired of these pathetic words, as you have already grown tired of other pathetic words, for instance "sin," "salvation," and "redemption."

And now, my friend, do not talk to me about the categorical imperative! That word tickles my ear, and I must laugh in spite of your presence and your seriousness. In this connection I recollect old Kant, who, as a punishment for having gained possession surreptitiously of the "thing in itself" also a very ludicrous affair! was imposed upon by the categorical imperative, and with that in his heart strayed back again to "God," the "soul," "freedom," and "immortality," like a fox which strays back into its cage: and it had been his strength and shrewdness which had broken open this cage! What? You admire the categorical imperative in you? This "persistency" of your so called moral judgment? This absoluteness of the feeling that "as I think on this matter, so must everyone think"? Admire rather your selfishness therein! And the blindness, paltriness, and modesty of your selfishness! For it is selfishness in a person to regard his judgment as universal law, and a blind, paltry and modest selfishness besides, because it betrays that you have not yet discovered yourself, that you have not yet created for yourself any personal, quite personal ideal: for this could never be the ideal of another, to say nothing of all, of everyone! He who still thinks that "each would have to act in this manner in this case," has not yet advanced half a dozen paces in self knowledge: otherwise he would know that there neither are, nor can be, similar actions, that every action that has been done, has been done in an entirely unique and inimitable manner, and that it will be the same with regard to all future actions; that all precepts of conduct (and even the most esoteric and subtle precepts of all moralities up to the present), apply only to the coarse exterior, that by means of them, indeed, a semblance of equality can be attained, but only a semblance, that in outlook and retrospect, every action is, and remains, an impenetrable affair, that our opinions of the "good," "noble" and "great" can never be proved by our actions, because no action is knowable, that our opinions, estimates, and tables of values are certainly among the most powerful levers in the mechanism of our actions, that in every single case, nevertheless, the law of their mechanism is untraceable.

Let us confine ourselves, therefore, to the purification of our opinions and appreciations, and to the construction of new tables of value of our own: we will, however, brood no longer over the" moral worth of our actions"! Yes, my friends! As regards the whole moral twaddle of people about one another, it is time to be disgusted with it! To sit in judgment morally ought to be opposed to our taste! Let us leave this nonsense and this bad taste to those who have nothing else to do, save to drag the past a little distance further through time, and who are never themselves the present, consequently to the many, to the majority! We, however, would seek to become what we are, the new, the unique, the m comparable, making laws for ourselves and creating ourselves! And for this purpose we must become the best students and discoverers of all the laws and necessities in the world. We must be physicists in order to be creators in that sense, whereas hitherto all appreciations and ideals have been based on ignorance of physics, or in contradiction thereto. And therefore, three cheers for physics! And still louder cheers for that which impels us to it our honesty.

I often feel like cheering aloud after reading that passage; it's one of my favorites.

EDIT: It seems like there's no standard way to divide the paragraphs, and the website I found the quote on had no paragraph breaks at all, so I just did it on my own by dividing it where it felt natural to me.