St. Paul tells us, Be ye angry and sin not. Anger is a venial sin if it does not proceed to action.
Certainly there is a place for righteous anger. Righteous anger hates the sin but not the sinner. Righteous anger is the justifiable detestation and opposition to sin that a man must have in his heart if he is to be saved. This kind of anger is best turned back upon the self. First, a man prays for the man whose sin has made him angry. Second, he looks into his own heart and finds examples of this sin in his own life. Men are best able to judge others' particular sins because they are fitted to locate the sin by its near perfect presence in their own hearts. Righteous indignation and anger should proceed only into action as prayer on the part of the one who has detected it.
I answer that, The movement of anger may be inordinate and sinful in two ways, as stated above (Article 2). First, on the part of the appetible object, as when one desires unjust revenge; and thus anger is a mortal sin in the point of its genus, because it is contrary to charity and justice.
Anger is a mortal sin when the subject is moved to seek retribution and retaliation against an object who has wronged him. ‘Vengeance is mine, saith the Lord.” (Deut. xxxii. 35) “Unjust” revenge is an exaggerated claim whereby the subject’s sense of hurt moves him to destroy the man who has become his enemy. Anger or Wrath here is contrary to both charity and justice. It is contrary to charity since it supersedes and extinguishes any sense of love towards a man. The object may indeed have made himself the subject's enemy. But this is no reason not to love his enemy in Christ and Christ in him. Anger or wrath is thus the vice that destroys its opposite virtue of charity. In another way wrath or anger destroys justice. Justice is giving to every man his due. Anger or Wrath is also contrary to justice. It is an expression of injustice. Rather than seeking a just solution to a disagreement, it resorts to violence of mind and body in order to punish an offender. Strictly speaking it is an exaggerated sensitive or appetitive response to an injury that ought to be resolved rationally. And even it the matter cannot be resolved rationally, the injured party is to endure the wrong with a justice that is merciful and kind, since this is the way of God towards man. God’s justice is merciful, and so too must the Christian’s be.
Nevertheless such like anger may happen to be a venial sin by reason of the imperfection of the act. This imperfection is considered either in relation to the subject desirous of vengeance, as when the movement of angerr forestalls the judgment of his reason; or in relation to the desired object, as when one desires to be avenged in a trifling matter, which should be deemed of no account, so that even if one proceeded to action, it would not be a mortal sin, for instance by pulling a child slightly by the hair, or by some other like action.
Sometimes anger is a venial sin because the subject acts impulsively, compulsively, and irrationally in the heat of the moment. In such a situation the subject is moved by bodily and appetitive irritability and discomfort. In another way the object may elicit from the subject a response of unpremeditated vengeance. The subject may wish to forestall any further offense from the object, and thus, for example, he might proceed to hit or pull the hair of an offending object. In neither case is the anger or wrath a mortal sin since it is not rationally contrived to injure the offending party excessively beyond the demands of justice.
Secondly, the movement of anger may be inordinate in the mode of being angry, for instance, if one be too fiercely angry inwardly, or if one exceed in the outward signs of anger. On this way anger is not a mortal sin in the point of its genus; yet it may happen to be a mortal sin, for instance if through the fierceness of his anger a man falls away from the love of God and his neighbor.
Anger is mortal if it becomes an inward habit of the mind and heart. It becomes mortal also if it becomes an external activity that is excessive establishes a sinful habit. So the sin becomes mortal because of its effect, as when anger moves a man to fall away from love of God and neighbor. Thus the effect or the activity draws a man by reason of repeated habits into the nature of the sin. Anger here is made a mortal sin by the activity that then draws the mind away from the rational good. So the habit overtakes the man and the man’s reason is then held captive to a habit that began through venial sin but has become mortal. So anger may become a mortal sin or habitual sin unto death if it is practiced and then formative of the sinner. Or it may become mortal if a man through reason desires vengeance against another excessively or inappropriately.
Reply to Objection 1. It does not follow from the passage quoted that all anger is a mortal sin, but that the foolish are killed spiritually by anger, because, through not checking the movement of anger by their reason, they fall into mortal sins, for instance by blaspheming God or by doing injury to their neighbor.
So the effect can lead to the development of the mortal sin. What begins in the appetitive and sensitive soul can overtake and overrule the rational soul. What begins in the body moves into the will. Once the habit is established in the heart or the will, it is then difficult for reason to overcome it. In this case a man acquires the sin that becomes worse the more it rules and governs his behavior towards God and neighbor.
Reply to Objection 2. Our Lord said this of anger, by way of addition to the words of the Law: Whosoever shall kill shall be in danger of the judgment. (Matthew v. 21). Consequently our Lord is speaking here of the movement of anger wherein a man desires the killing or any grave injury of his neighbor: and should the consent of reason be given to this desire, without doubt it will be a mortal sin.
A man may kill another man literally, spiritually, or both. The point is that reason is used to contrive or assent to the murder of another. That reason assents to it is evidence that the subject is perfecting mortal sin since he does not love his neighbor.
Reply to Objection 3. In the case where anger is contrary to charity, it is a mortal sin, but it is not always so, as appears from what we have said.
To be a mortal sin anger or wrath must work clean contrary to charity and justice as mercy. The mortal nature of the sin is evidenced in a real destruction of the mind’s ability to love and hope for the transformation of another. So the mortal sin or anger or wrath cuts a man off from God and his neighbor. It can be overcome only by a deep sense of God's merciful charity and His long-suffering and lenient justice. Let us embrace both as we seek to overcome all anger and wrath.
St. Michael and All Angels Sermons