I haven’t read Lacan’s article connecting Sade and Kant, but if I remember Žižek’s discussion of it, the connection is between Kant’s insistence that duty is more important than benevolence, and Sade’s dutiful pursuit of malevolence; as in this passage from the Critique of Practical Reason:
It is very beautiful to do good to human beings from love for them and from sympathetic benevolence, or to be just from love of order; but this is not yet the genuine moral maxim of our conduct, the maxim befitting our position among rational beings as human beings, when we presume with proud conceit, like volunteers, not to trouble ourselves about the thought of duty and, as independent of command, to want to do of our own pleasure what we think we need no command to do. (82)
I wonder, though, if the apparently shocking connection to Sade isn’t less illuminating than the more obvious connection to Masoch. This connection starts off absolutely straightforward, in Kant’s connection of duty to submission:
Duty! Sublime and mighty name that embraces nothing charming or insinuating, but requires submission, and yet does not seek to move the will by threatening anything that would arouse natural aversion or terror in the mind, but merely holds forth a law which of itself finds entrance into the mind, and yet gains reluctant reverence (though not always obedience), a law before which all inclinations are dumb, even though they secretly work against it; what origin is there worthy of you, and where is to be found the root of your noble descent which proudly rejects all kindred with the inclinations, descent from which is the indispensable condition of that worth which human beings along can give themselves? (86)
Kant prefers this duty to “a spontaneous goodness of heart that needs neither spur nor bridle” (85). But using this quote on its own to put Kant in a gimp mask misses the point of what the “spur and bridle” are doing, for Kant. The important point is that “duty” can both “find entrance into the mind” and allow human beings to “give themselves” worth. The famously paradoxical part of Kant’s moral theory is that all this submission is not about bondage, but is, in Kant’s system, the only possible form of freedom. Practical reason produces the subject’s relation to the world
not indeed with a view to any theoretic employment of it, i.e., so as to bring the manifold of (sensible) intuition under one consciousness a priori; but only to subject the manifold of desires to the unity of consciousness of a practical reason, giving it commands in the moral law, i.e., to a pure will a priori. (65)
The empirical consciousness unites our diverse sense impressions to produce objects which we can know; but this knowledge is always determined by the world, and hence cannot be free. The spurs and bridles are necessary to unite our diverse desires to produce an agent capable of changing the world. Kant’s masochism, then, is a rigorous process of self-fashioning, a punishment we inflict on ourselves to create a different “us” that could be free.