Essay II of Genealogy of Morals is entitled “Guilt, Bad Conscience, and Related Matters”.
Nietzsche identifies “the contractual relationship between creditor and debtor” as the sociological source and origin of issues like injury, guilt, and punishment (section 4).
Section 20 states that “the consciousness of being in debt to the gods” increases in direct proportionaly to the influence in society of the god concept. Hence one would conjecture that the decline of the importance of the god concept in society also diminuishes the strength of the consciousness of guilt.
According to section 21 any assessment of guilt and duty from a moral point of view has to consider the human conscience. Nietzsche states that contrary to expectations the dominance of these concepts did not diminuish even for humans who no longer believe in god. The terms guilt and duty are no longer part of a contractual relation to god but move into the human conscience and become moral terms.
the aim now is to preclude pessimistically, once and for all, the prospect of a final discharge....
says: With the decline of the religious worldview a final discharge by god is no longer possible, which aggravates the human situation.
The bad conscience attacks the creditor but – according to Nietzsche – also the debtor.
But also to me it remains unclear who is considered the debtor after the decline of the god concept. Hence I have to join you last question.