In a web essay, Jean-Michel Rabaté traces the background to Lacan’s notorious coupling of Kant and Sade. One mediating figure is Freud. In an essay on the “economic problem of masochism,” Freud linked the Kantian categorical imperative with the cruel demands of the super-ego:

“This super-ego is in fact just as much a representative of the id as of the outer world. It originated through the introjection into the ego of the first objects of the libidinal impulses in the id, namely,
the two parents, by which process the relation to them was desexualized, that is underwent a deflection from direct sexual aims.
Only in this way was it possible for the child to overcome the
Oedipus-complex . . . .

“Now the super-ego has retained essential features of the introjected persons, namely their power, their severity, their tendency to watch over and to punish . . . . The super-ego, the conscience at work in it, can then become harsh, cruel and inexorable against the ego which is in its charge. The categorical imperative of Kant is thus a direct inheritance from the Oedipus complex.”

This creates a perverse couple: The sadistic law/super-ego meets the masochistic ego. Freud sees this combination in “Russian character types” who violate moral rules in order to suffer the pangs of conscience.

Hegel is another precursor, interestingly accusing Kant of a kind of Judaizing. Rabaté writes:

“Before Freud, Hegel’s critique of Kant’s version of morality in ‘The Spirit of Christianity’ (1798-99), provides an early negative appraisal of Kantian morality. For Hegel, Kant appears as the modern
successor of Jewish law-givers like Abraham and Moses who ‘exercised
their dominion mercilessly with the most revolting and harshest
tyranny . . . utterly extirpating all life; for it is only over
death that unity hovers.’ A real stranger to everything including
love, Abraham takes the whole world as his opposite, and he creates
the picture of a terrifying God who is also a merciless stranger and
the Master of a people he reduces to religious slavery. Hegel agrees
with Freud in that he sees Moses as more Egyptian than the Egyptians,
and the founder of an ‘oriental’ system of absolute domination:
‘Moses sealed his legislation with an oriental beautiful threat of
the loss of all pleasure and all fortune. He brought before the
slavish spirit the image of itself, namely, the terror of physical
force.’ Hegel’s ‘Spirit of Christianity’ thus sketches the theological genesis of the castrating Father, anticipating on Moses and Monotheism by some hundred and fifty years.”

For all of Kant’s desire to cling to the spirit rather than the letter, his categorical imperative cannot help but be letter-oriented: “Similarly, Kant is accused by Hegel of importing a Jewish formalism or ‘positivity’ of the law into philosophy; For Hegel, Kant’s misinterprets the Christian commandment ‘Love God above everything
and thy neighbor as thyself’ as a ‘command requiring respect for a law which commands love.’ This ‘reduction’ of ‘love’ to a ‘command’ is a great perversion according to Hegel ‘because in love all thought of duties vanish.’ In these early texts, Hegel extols Jesus for being able to raise love above any type of morality. Jesus does not praise reverence for the laws but announces a self-annulling love, a love that ‘exhibits that which fulfills the law but annuls it as law and so is something higher than obedience to law and makes law superfluous.’”