Quintilian, The Education of the Orator, 3.8.44-47:
Meanwhile, if you are trying to persuade someone good to do something shameful, remember not to try to persuade him on the grounds that it is shameful-- the way that some declaimers urge Sextus Pompey toward piracy with the very excuse that he is already dishonest and cruel. Rather, such ugly matters need to be given an angle. This obtains even with evil people: no one is so evil that he wants to seem so. Thus in Sallust, Catiline speaks so as to seem to be attempting the worst crime not out of wickedness but out of a sense of outrage; thus in Varius’s Thyestes, Atreus says: “Now I accept, now I am forced to commit the most unspeakable.” This ambition, as it were, has to be protected all the more in those audiences who are concerned about their reputation. That’s why when we advise Cicero to plead with Antony, even on the condition that he burn the Philippics (Antony promises to spare him if he does), we will not appeal to Cicero’s desire to live (because if this is a strong motive in his mind, it will be strong even if we don’t mention it). Rather, we exhort him to preserve himself for the good of the state. He needs this kind of excuse so that he can be unashamed to make such a plea. Similarly, when we are persuading Caesar to take absolute power, we assert that the state cannot be stable unless a single person rules it. For someone who is considering a nefarious deed looks chiefly for how he can least appear to be committing a crime.