Habitual anger is poison to the soul. If not countered by the antidote of humble prayer, it will surely lead to spiritual death.
Some, on hearing this, respond, "But we have good reason to be angry!"
No one would dispute that there is plenty going on today that could enkindle a zealous passion to correct injustice and vice. But where does that leave us? With good reason to do something that, if not countered by humble prayer, will surely lead to spiritual death.
This paradox can be resolved along two lines.
First, the action of another that constitutes our "good reason to be angry" is not really, by itself, a good reason to be angry. Such an action -- the injustice we desire to avenge -- is necessary to be justly angry, but it is not sufficient. Just anger must accord with reason, and the reason involved is not only, "This is a very bad thing!" but, "This is what I can do to correct it!" It is not in accord with reason to get angry at the burning of the Library of Alexandria, because that's an injustice that cannot now be avenged.
Even given an injustice that can still be avenged, the questions remain, "Can I avenge it?" "Can I avenge it in this way?" Only if the answers to these questions are yes is the anger that commits me to act in this way just.
The second line of resolution is the distinction between an act of anger and the habit of anger. I am not too concerned about individual acts of anger (my own aside); some aren't sinful, and those that are ... well, that's what the confessional is for (to say nothing of Matthew 7:3-5).
But the habit of anger is a different matter. The habit of anger is one of the Seven Capital Vices (a vice, remember, is simply an evil habit). Its daughters, per St. Gregory the Great, are quarreling, swelling of the mind, contumely, clamor, indignation and blasphemy. In my judgment, the habit of anger is a loudly sounded note of the Church in the United States at this time, and having its daughters running around freely makes it harder for the faithful to conform ourselves to Christ and to fulfill the Father's will for us in this life.
Moreover, too often this vice is regarded as a virtue. People fail to distinguish between necessary and sufficient causes to be virtuously angry. It's sort of the dual of the "ends justify means" argument, a "cause justifies means" argument that whatever I do must be good because I am responding to something clearly evil.
On top of that, they fail to see what habitual anger -- even and perhaps especially anger at true injustice -- does to a human soul, how it shrivels the heart and blinds the reason.
Many chemical solvents are poisonous, even corrosive. We know that, and we treat them with care. We store them appropriately, take them out only for a particular use, follow the directions on the label, clean up thoroughly, then return the solvents to safe storage when we're done. If we're not completely oblivious, we're well aware of what might happen if we get careless.
Yet there are those who pick up anger, as poisonous and corrosive to the soul as high molar acids are to the body, when they wake and never lay it down throughout the day. They clutch it tight as they fall asleep. What would we say to someone who slept with a bottle of concentrated sulfuric acid, who carried it about with him during the day in search of things to dissolve with it?