Sinful Anger

“Anger as a deadly sin is ‘a disorderly outburst of emotion connected with the inordinate desire for revenge.’ . . . It is likely to be accompanied by surliness of heart, by malice aforethought, and above all by the determination to take vengeance.” The Catechism of the Catholic Church contains a similar description:

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.” (no. 2302)

This is different from the feeling of anger, which is not sinful in itself. That anger is defined as “a strong feeling of displeasure or hostility.” We can’t control when we will feel angry, since that depends on events that occur outside of us. But we can control what we do about the feeling.
As the Catechism of the Catholic Church teaches, the feeling of anger is one of the passions. “In the passions, as movements of the sensitive appetite, there is neither moral good nor evil. But insofar as they engage reason and will, there is moral good or evil in them” (no. 1773). So it is how we act based on this feeling that determines whether we sin or not. We find also in the Catechism that we should be moved to do good not only by our will but by our heart as well. In other words, we must attempt to convert our very feelings to be fully virtuous (no. 1775).
St. Paul mentions outbursts of anger along with sev­eral other sins, including fornication, jealousy, enmity, and strife. He concludes with this warning: “I warn you as I warned you before, that those who do such things will not inherit the kingdom of God” (Gal. 5:20–21). What could be further from Christ’s command in the Sermon on the Mount, to “love your enemies and pray for those who persecute you” (Matt. 5:43)? He said, “I say to you that every one who is angry with his brother shall be liable to judgment; whoever insults his brother shall be liable to the council, and whoever says, ‘You fool!’ shall be liable to the fire of hell” (Matt. 5:22).
We can suppress our anger; or we can express it by sabotaging the efforts of those who caused it; or we can express it in an irrational tirade of bad words and insults; or we can express our anger rationally — or at least process it rationally. If we merely suppress an angry feeling, it will go down into our subconscious and wait for a chance to explode. And it will explode. It is better to do something constructive with it.
Link