The above becomes (again) an absolute prohibition of the questioning of the origin & legitimacy of the state power in Kant's political philosophy:
***** A people should not _inquire_ with any practical aim in view into the origin of the supreme authority to which it is subject, that is, a subject _ought not to reason subtly_ for the sake of action about the origin of this authority, as a right that can still be called into question (_jus controversum_) with regard to the obedience he owes it. For, since a people must be regarded as already united under a general legislative will in order to judge with rightful force about the supreme authority (_summum imperium_), it cannot and may not judge otherwise than as the present head of state (_summus imperans_) wills it to. Whether a state began with an actual contract of submission (_pacta subiectionis civils_) as a fact, or whether power came first and law arrived only afterwards, or even whether they should have followed in this order: for a people already subject to civil law these subtle reasonings are altogether pointless and, moreover, threaten a state with danger. If a subject, having pondered over the ultimate origin of the authority now ruling, wanted to resist this authority, he would be punished, got rid of, or expelled (as an outlaw, _exlex_) in accordance with the laws of this authority, that is, with every right. A law that is so holy (inviolable) that it is already a crime even to call it in doubt _in a practical way_, and so to suspend its effect for a moment, is thought as if it must have arisen not from human beings but from some highest, flawless lawgiver; and that is what the saying "All authority is from God" means. (_The Metaphysics of Morals_) *****
The 'ultimate evil' appears in a footnote to _The Metaphysics of Morals_:
***** Now the criminal can commit his misdeed either by adopting a maxim based on an assumed objective law (as if it were universally valid), or merely as an exception to the rule (by exempting himself from it as the occasion requires). In the _latter_ case, he merely _deviates_ (albeit deliberately) from the law, for he may at the same time deplore his own transgression and simply wish to get round the law without formally terminating his obedience to it. But in the _former_ case, he rejects the authority of the law itself..., and makes it his rule to act in opposition to it; it is actually _contrary_ to the law..., or...diametrically opposed to it as a contradiction.... So far as can be seen, it is impossible for men to commit a crime of such formal and completely futile malice, although no system of morality should omit to consider it, if only as a pure idea representing ultimate evil.
Thus the reason why the thought of the formal execution of a monarch _by his people_ inspires us with dread is that, while his _murder_ must be regarded merely as an exception to the rule which the people have taken as their maxim, his _execution_ must be seen as a complete _reversal_ of the principles which govern the relationship between the sovereign and the people. For it amounts to making the people, who owe their existence purely to the legislation of the sovereign, into rulers over the sovereign, thereby brazenly adopting violence as a deliberate principle and exalting it above the most sacred canons of right. And this, like an abyss which engulfs everything beyond hope of return, is an act of suicide by the state, and it would seem to be a crime for which there can be no atonement. There are therefore grounds for assuming that agreements to perform such executions do not really proceed from any supposed principle of right, but from the people's fear of revenge from the state if it should ever recover, and that such formalities are introduced only in order to give the deed an air of penal justice and of _rightful procedure_ (with which murder, on the other hand, could not be reconciled). But this disguise is futile, since any such presumption on the part of the people is more atrocious than murder itself, for it in fact embodies a principle which must make it impossible for an overthrown state to be reconstituted. *****
So, the ultimate evil is eminently thinkable, theoretically or historically (Kant himself mentions the fate of Charles I and Louis XVI in the same footnote); it's just that thinking of it (the ultimate evil = a revolutionary rejection of the authority of the law) destroys the Kantian system of morality and political philosophy.
Yoshie