Objection 1. It would seem that the sin of respect of persons does not take place in judicial sentences. For respect of persons is opposed to distributive justice, as stated above (Article 1): whereas judicial sentences seem to pertain chiefly to commutative justice. Therefore respect of persons does not take place in judicial sentences.
Objection 2. Further, penalties are inflicted according to a sentence. Now it is not a sin to respect persons in pronouncing penalties, since a heavier punishment is inflicted on one who injures the person of a prince than on one who injures the person of others. Therefore respect of persons does not take place in judicial sentences.
Objection 3. Further, it is written (Sirach 4:10): "In judging be merciful to the fatherless." But this seems to imply respect of the person of the needy. Therefore in judicial sentences respect of persons is not a sin.
On the contrary, It is written (Proverbs 18:5): "It is not good to accept the person in judgment [Vulgate: 'It is not good to accept the person of the wicked, to decline from the truth of judgment.']."
I answer that, As stated above (Question 60, Article 1), judgment is an act of justice, in as much as the judge restores to the equality of justice, those things which may cause an opposite inequality. Now respect of persons involves a certain inequality, in so far as something is allotted to a person out of that proportion to him in which the equality of justice consists. Wherefore it is evident that judgment is rendered corrupt by respect of persons.
Reply to Objection 1. A judgment may be looked at in two ways. First, in view of the thing judged, and in this way judgment is common to commutative and distributive justice: because it may be decided by judgment how some common good is to be distributed among many, and how one person is to restore to another what he has taken from him. Secondly, it may be considered in view of the form of judgment, in as much as, even in commutative justice, the judge takes from one and gives to another, and this belongs to distributive justice. On this way respect of persons may take place in any judgment.
Reply to Objection 2. When a person is more severely punished on account of a crime committed against a greater person, there is no respect of persons, because the very difference of persons causes, in that case, a diversity of things, as stated above (58, 10, ad 3; 61, 2, ad 3).
Reply to Objection 3. In pronouncing judgment one ought to succor the needy as far as possible, yet without prejudice to justice: else the saying of Exodus 23:3 would apply: "Neither shalt thou favor a poor man in judgment."
Source: Summa Theologica (New Advent)