M. Tullius Cicero Cum Populo Gratias Egit 21

[21] And although he, in his angry mind, laboured for nothing but avenging himself on his enemies, I will only think of my enemies as much as the republic herself allows me. 9
  Lastly, O Romans, since they are altogether four classes of men who injured me,—one of them, those who were most hostile to me out of hatred to the republic, because I had preserved it against their will; another, those who most wickedly betrayed me under pretence of friendship; a third, those who envied my credit and dignity, because they, from their laziness, could not obtain the same honours; the fourth was composed of those men who, while they ought to have been guardians of the republic, sold (as far as was in their power) my safety, the constitution of the state, and the dignity of its empire; I will revenge myself on each class in proportion as I have been challenged by each—on wicked citizens, by conducting the republic successfully; on my perfidious friends, by trusting them in nothing, and taking every sort of precaution against them; on the envious, by obeying virtue and glory; on the buyers of provinces, by recalling them home, and by exacting from them an account of their conduct in those provinces.