4 The wisdom of these regulations is more evident as one examines the absurd justification or excuses for the inhuman custom of dueling. The generally held argument that this sort of struggle washes away, as it were, the stains that calumny or insult has brought upon the honor of citizens surely can deceive no one but a madman. Even if the challenger of a duel is the victor, all reasonable persons will admit that the outcome simply proves he is the better man in strength or in handling a weapon, not the better man in honor. But if he falls in the combat, does he not prove by the same token how absurd is this way of protecting his honor? Few there are, we believe, who commit this crime deceived by erroneous opinion. It is, to be sure, the desire of revenge that impels passionate and arrogant men to seek satisfaction. God commands all men to love each other in brotherly love and forbids them to ever violate anyone; he condemns revenge as a deadly sin and reserves to himself the right of expiation. If people could restrain their passion and submit to God, they would easily abandon the monstrous custom of dueling.
Source: Pastoralis Officii (Vatican.va)