Theophylact of Ohrid

Do not speak evil of anyone, even if someone acts badly — that is, do not reproach. Our mouths should be clean from reproach. Whether justly or falsely — it is not for us to judge. Consider what danger there may be. To show all meekness to all men — to both Jews and Greeks, and to those who cause harm, and to the unjust.

Commentary on Titus ↗

Oecumenius

To speak evil of no one, to be peaceable, gentle, showing all humility to all people. to speak evil of no one. Even if he acts badly, even if he makes a mistake in something. For our mouth must generally be free from insult. "For you say, 'Why do you judge your brother?" (Rom.14:10" Why do you alter the judgment of God? For he calls the insult against his brother blasphemy. showing all humility to all people. Both Jews and Greeks, and friends and enemies, and those who wrong you and those who harass.

Commentary On Titus ↗

Cyprian

That we must not use detraction. In Solomon, in the Proverbs: "Love not to detract, lest thou be taken away." Also in the forty-ninth Psalm: "Thou sattest, and spakest against thy brother; and against the son of thy mother thou placedst a stumbling-block." Also in the Epistle of Paul to the Colossians: "To speak ill of no man, nor to be litigious."

Treatise XII. Three Books of Testimonies Against the Jews. ↗

Thomas Aquinas

Then when he says, speak evil of no man, he shows how Titus should act toward his equals: first, in regard to avoiding evil; second, in regard to doing good, at but gentle. He warns them particularly about words, because in the early Church few sinned in deed. By words one might sin by offending the person of another, if he speaks insultingly; hence he says, speak evil of no man. But blasphemy is charging God with a crime; therefore, there cannot be blasphemy against one's neighbor. …

Commentary on Titus ↗