10 Now the third condition for gaining happiness is not to sit in the seat of pestilence. The Pharisees sat as teachers in Moses' seat, and Pilate sat in the seat of judgment: of what seat then are we to consider the occupation pestilential? Not surely of that of Moses, for it is the occupants of the seat and not the occupation of it that the Lord condemns when He says: The Scribes and Pharisees sit on Moses' seat; whatsoever they bid you do, that do; but do not ye after their work. The occupation of that seat is not pestilential, to which obedience is enjoined by the Lord's own word. That then must be really pestilential, the infection of which Pilate sought to avoid by washing his hands. For many, even God-fearing men, are led astray by the canvassing for worldly honours; and desire to administer the law of the courts, though they are bound by those of the Church.
But although they bring to the discharge of their duties a religious intention, as is shown by their merciful and upright demeanour, still they cannot escape a certain contagious infection arising from the business in which their life is spent. For the conduct of civil cases does not suffer them to be true to the holy principles of the Church's law, even though they wish it. And without abandoning their pious purpose they are compelled, against their will, by the necessary conditions of the seat they have won, to use, at one time invective, at another, insult, at another, punishment; and their very position makes them authors as well as victims of the necessity which constrains them, their system being as it were impregnated with the infection. Hence this title, the seat of pestilence, by which the Prophet describes their seat, because by its infection it poisons the very will of the religiously minded.
Source: Homilies on the Psalms (New Advent)