Objection 1. It would seem that it cannot be lawful to be angry. For Jerome in his exposition on Matthew 5:22, "Whosoever is angry with his brother," etc. says: "Some codices add 'without cause.' However, in the genuine codices the sentence is unqualified, and anger is forbidden altogether." Therefore it is nowise lawful to be angry.
Objection 2. Further, according to Dionysius (Div. Nom. iv) "The soul's evil is to be without reason." Now anger is always without reason: for the Philosopher says (Ethic. vii, 6) that "anger does not listen perfectly to reason"; and Gregory says (Moral. v, 45) that "when anger sunders the tranquil surface of the soul, it mangles and rends it by its riot"; and Cassian says (De Inst. Caenob. viii, 6): "From whatever cause it arises, the angry passion boils over and blinds the eye of the mind." Therefore it is always evil to be angry.
Objection 3. Further, anger is "desire for vengeance" [Aristotle, Rhet. ii, 2 according to a gloss on Leviticus 19:17, "Thou shalt not hate thy brother in thy heart." Now it would seem unlawful to desire vengeance, since this should be left to God, according to Deuteronomy 32:35, "Revenge is Mine." Therefore it would seem that to be angry is always an evil.
Objection 4. Further, all that makes us depart from likeness to God is evil. Now anger always makes us depart from likeness to God, since God judges with tranquillity according to Wisdom 12:18. Therefore to be angry is always an evil.
On the contrary, Chrysostom [Hom. xi in the Opus Imperfectum, falsely ascribed to St. John Chrysostom] says: "He that is angry without cause, shall be in danger; but he that is angry with cause, shall not be in danger: for without anger, teaching will be useless, judgments unstable, crimes unchecked." Therefore to be angry is not always an evil.
I answer that, Properly speaking anger is a passion of the sensitive appetite, and gives its name to the irascible power, as stated above (I-II, 46, 1) when we were treating of the passions. Now with regard to the passions of the soul, it is to be observed that evil may be found in them in two ways. First by reason of the passion's very species, which is derived from the passion's object. Thus envy, in respect of its species, denotes an evil, since it is displeasure at another's good, and such displeasure is in itself contrary to reason: wherefore, as the Philosopher remarks (Ethic. ii, 6), "the very mention of envy denotes something evil." Now this does not apply to anger, which is the desire for revenge, since revenge may be desired both well and ill. Secondly, evil is found in a passion in respect of the passion's quantity, that is in respect of its excess or deficiency; and thus evil may be found in anger, when, to wit, one is angry, more or less than right reason demands. But if one is angry in accordance with right reason, one's anger is deserving of praise.
Reply to Objection 1. The Stoics designated anger and all the other passions as emotions opposed to the order of reason; and accordingly they deemed anger and all other passions to be evil, as stated above (I-II, 24, 2) when we were treating of the passions. It is in this sense that Jerome considers anger; for he speaks of the anger whereby one is angry with one's neighbor, with the intent of doing him a wrong.--But, according to the Peripatetics, to whose opinion Augustine inclines (De Civ. Dei ix, 4), anger and the other passions of the soul are movements of the sensitive appetite, whether they be moderated or not, according to reason: and in this sense anger is not always evil.
Reply to Objection 2. Anger may stand in a twofold relation to reason. First, antecedently; in this way it withdraws reason from its rectitude, and has therefore the character of evil. Secondly, consequently, inasmuch as the movement of the sensitive appetite is directed against vice and in accordance with reason, this anger is good, and is called "zealous anger." Wherefore Gregory says (Moral. v, 45): "We must beware lest, when we use anger as an instrument of virtue, it overrule the mind, and go before it as its mistress, instead of following in reason's train, ever ready, as its handmaid, to obey." This latter anger, although it hinder somewhat the judgment of reason in the execution of the act, does not destroy the rectitude of reason. Hence Gregory says (Moral. v, 45) that "zealous anger troubles the eye of reason, whereas sinful anger blinds it." Nor is it incompatible with virtue that the deliberation of reason be interrupted in the execution of what reason has deliberated: since art also would be hindered in its act, if it were to deliberate about what has to be done, while having to act.
Reply to Objection 3. It is unlawful to desire vengeance considered as evil to the man who is to be punished, but it is praiseworthy to desire vengeance as a corrective of vice and for the good of justice; and to this the sensitive appetite can tend, in so far as it is moved thereto by the reason: and when revenge is taken in accordance with the order of judgment, it is God's work, since he who has power to punish "is God's minister," as stated in Romans 13:4.
Reply to Objection 4. We can and ought to be like to God in the desire for good; but we cannot be altogether likened to Him in the mode of our desire, since in God there is no sensitive appetite, as in us, the movement of which has to obey reason. Wherefore Gregory says (Moral. v, 45) that "anger is more firmly erect in withstanding vice, when it bows to the command of reason."
Source: Summa Theologica (New Advent)