Catholic Encyclopedia (1913)/Anger
The desire of vengeance. Its ethical rating depends upon the quality of the vengeance and the quantity of the passion. When these are in conformity with the prescriptions of balanced reason, anger is not a sin. It is rather a praiseworthy thing and justifiable with a proper zeal. It becomes sinful when it is sought to wreak vengeance upon one who has not deserved it, or to a greater extent than it has been deserved, or in conflict with the dispositions of law, or from an improper motive. The sin is then in a general sense mortal as being opposed to justice and charity. It may, however, be venial because the punishment aimed at is but a trifling one or because of lack of full deliberation. Likewise, anger is sinful when there is an undue vehemence in the passion itself, whether inwardly or outwardly. Ordinarily it is then accounted a venial sin unless the excess be so great as to go counter seriously to the love of God or of one's neighbour.
JOSEPH F. DELANY