Quintilian, Institutio Oratoria 3.8
“Meanwhile, should anyone persuade a good man of dishonest, let them remember not to persuade them as dishonest thing, as some declaimers say that Sextus Pompeius should turn to piracy precisely because it is shameful and cruel. Rather, one should give unpleasant acts a certain favorable coloring even among the wicked, because no one is so evil that they wish to appear to be so. Thus Catiline speaks in Sallust in such a way that he seems to undertake the most wicked crime not from malice but from indignation. Thus Atreus in Varius says, ‘Now I will undertake the most unspeakable things, now I am compelled to do so.’ How much more should this display be maintained by those who care about their reputation! For this reason we will even give Cicero the counsel to entreat with Anthony, or even to burn his Philippics if Antony promised to leave him alive, and we will not attribute this to his desire to stay alive (even if this is his chief motive, it will have its force as we remain silent on the matter. But we will urge the Republic to save itself. It is necessary for it on this occasion that one not be ashamed of such prayers. We will even affirm to Julius Caesar, as we persuade him to take the throne, that the republic can not stand unless it be ruled by one man. For one who deliberates about a nefarious deed only asks how he might appear to have sinned the least.”
Interim si quis bono inhonesta suadebit, meminerit non suadere tamquam inhonesta, ut quidam declamatores Sextum Pompeium ad piraticam propter hoc ipsum, quod turpis et crudelis sit, inpellunt, sed dandus illis deformibus color idque etiam apud malos: neque enim quisquam est tam malus ut videri velit. XLV. Sic Catilina apud Sallustium loquitur ut rem scelestissimam non malitia sed indignatione videatur audere, sic Atreus apud Varium “iam fero” inquit “infandissima, iam facere cogor”. Quanto magis eis quibus cura famae fuit conservandus est hic vel ambitus. XLVI. Quare et cum Ciceroni dabimus consilium ut Antonium roget, vel etiam ut Philippicas, ita vitam pollicente eo, exurat, non cupiditatem lucis adlegabimus (haec enim si valet in animo eius, tacentibus quoque nobis valet), XLVII. sed ut se rei publicae servet hortabimur – hac illi opus est occasione, ne eum talium precum pudeat: et C. Caesari suadentes regnum adfirmabimus stare iam rem publicam nisi uno regente non posse. Nam qui de re nefaria deliberat id solum quaerit, quo modo quam minimum peccare videatur.