William Makepeace Thackeray knew it, and wrote thus in “Vanity Fair,” (1848) chapter XVIII:
“When one man has been under very remarkable obligations to another, with whom he subsequently quarrels, a common sense of decency, as it were, makes of the former a much severer enemy than a mere stranger would be. To account for your own hard-heartedness and ingratitude in such a case, you are bound to prove the other party’s crime. It is not that you are selfish, brutal, and angry at the failure of a speculation—no, no—it is that your partner has led you into it by the basest treachery and with the most sinister motives. From a mere sense of consistency, a persecutor is bound to show that the fallen man is a villain—otherwise he, the persecutor, is a wretch himself. And as a general rule, which may make all creditors who are inclined to be severe pretty comfortable in their minds, no men embarrassed are altogether honest, very likely. They conceal something; they exaggerate chances of good luck; hide away the real state of affairs; say that things are flourishing when they are hopeless, keep a smiling face (a dreary smile it is) upon the verge of bankruptcy—are ready to lay hold of any pretext for delay or of any money, so as to stave off the inevitable ruin a few days longer. “Down with such dishonesty,” says the creditor in triumph, and reviles his sinking enemy. “You fool, why do you catch at a straw?” calm good sense says to the man that is drowning. “You villain, why do you shrink from plunging into the irretrievable Gazette?” says prosperity to the poor devil battling in that black gulf. Who has not remarked the readiness with which the closest of friends and honestest of men suspect and accuse each other of cheating when they fall out on money matters? Everybody does it. Everybody is right, I suppose, and the world is a rogue.”
David Mamet knows it. From “The Spanish Prisoner,” his 1997 movie:
Jimmy Dell: I think you’ll find that if what you’ve done for them is as valuable as you say it is, if they are indebted to you morally but not legally, my experience is they will give you nothing, and they will begin to act cruelly toward you.
Joe Ross: Why?
Jimmy Dell: To suppress their guilt.
Nicolás Gómez Dávila knew it. From “Escolios a un texto implícito” (ed. 2009) (the translation is mine):
Those who betray us will never forgive us for their betrayal.