For a sin to be a mortal sin, there are always three conditions that have to be met:
- Grave Matter: The act itself is intrinsically evil and immoral
- Full Knowledge: The person must know that what they’re doing or planning to do is evil and immoral.
- Deliberate Consent: The person must freely choose to commit the act or plan to do it.
Reading what the catechism says about anger could be helpful. It says:
CCC 2302 “By recalling the commandment, “You shall not kill,” our Lord asked for peace of heart and denounced murderous anger and hatred as immoral.
Anger is a desire for revenge. “To desire vengeance in order to do evil to someone who should be punished is illicit,” but it is praiseworthy to impose restitution “to correct vices and maintain justice.” If anger reaches the point of a deliberate desire to kill or seriously wound a neighbor, it is gravely against charity; it is a mortal sin. The Lord says, “Everyone who is angry with his brother shall be liable to judgment.
So, anger is an emotion, when it is directed to pay back evil for evil, to revenge a wrong done to someone, it is a sin. If it is directed simply to correct, defend ones good and that of others, is it praiseworthy. An example would be when Jesus drove out those money changers in the temple (Mt 21:12). It is then a mortal sin when it meets the three conditions earlier mentioned otherwise it is a venial sin. Anger that intends to destroy or cause harm to others. Anger that has nothing good to achieve is outrightly sinful.
Since anger an emotion, the devil can quickly use it against us. So, St. Paul cautions us in Ephesians 4:26 saying “And ‘don’t sin by letting anger control you.’ Don’t let the sun go down while you are still angry.” Been angry because something is going wrong is one thing but beginning to wish bad things on others is sinful.
So, anger is sinful only when the intended or actual effect of it is bad. It is a good thing when helps to correct vices and maintain justice.