“Our fourth struggle is against the demon of anger. We must, with God’s help, eradicate his deadly poison from the depths of our souls. So long as he dwells in our hearts and blinds the eyes of the heart with his sombre disorders, we can neither discriminate what is for our good, nor achieve spiritual knowledge, nor fulfill our good intentions, nor participate in true life: and our intellect will remain impervious to the contemplation of the true, divine light; for it is written, ‘For my eye is troubled because of anger’ (Ps. 6:7. LXX).”
St John Cassian

The evil of anger for St John Cassian is that it blinds our souls and cuts us off from true life. ‘Anger is a desire for revenge.’ [CCC no. 2302] That is, it is to desire destruction or harm upon someone or something in response to a perceived injury received. It therefore blinds our souls because we see the other as evil/as an enemy, when in truth and in God’s eyes they are fundamentally good and beloved. It cuts us off from true life because we set ourselves against God’s beloved, and therefore against God Himself. Experience confirms that anger destroys reason and opposes prayer, without fail. I am sure we have all experienced “seeing red” and doing something we shouldn’t have.
Righteous anger?
“Listen to what St Paul enjoins: ‘Rid yourselves of all bitterness, wrath, anger, clamour, evil speaking and all malice’ (Eph. 4:31). In saying ‘all’ he leaves no excuse for regarding any anger as necessary or reasonable. If you want to correct your brother when he is doing wrong or to punish him, you must try to keep yourself calm; otherwise you yourself may catch the sickness you are seeking to cure and you may find that the words of the Gospel now apply to you: ‘Physician, heal yourself’ (Luke 4:23), or ‘Why do you look at the speck of dust in your brother’s eye, and not notice the rafter in your own eye?’ (Matt. 7:3).”
St John Cassian
St John is unequivocal: the only place for anger is against our own sins. However, the idea of righteous anger goes back at least to St Thomas Aquinas, and so deserves respectful consideration. What does St Thomas say, and can we reconcile the two saints?
“It is unlawful to desire vengeance considered as evil to the man who is to be punished, but it is praiseworthy to desire vengeance as a corrective of vice and for the good of justice”
ST II-II, Q. 158, art.1, reply obj.3
The difference between St Thomas and St John is just that St Thomas is willing to call this anger and St John is not. This desire for justice, for restitution and correction of vice, is utterly different to the desire for vengeance upon an enemy, to the point where we need another word for it. Righteous anger is as different to anger as chaste sexual desires are to lust.
The point I would like to emphasise here is, that we ought to keep calm even when we must correct or confront a brother. We may feel outrage over the transgression, but we must be calm and contain ourselves, and insist upon viewing our brother as a brother, and if we must confront a brother we must do so in charity.
The purpose of anger
“Our incensive power can be used in a way that is according to nature only when turned against our own impassioned or self-indulgent thoughts. This is what the Prophet teaches us when he says: ‘Be angry, and do not sin’ (Ps. 4:4. LXX) – that is, be angry with your own passions and with your malicious thoughts, and do not sin by carrying out their suggestions. What follows clearly confirms this interpretation: ‘As you lie in bed, repent of what you say in your heart’ (Ps. 4:4. LXX) – that is, when malicious thoughts enter your heart, expel them with anger, and then turn to compunction and repentance as if your soul were resting in a bed of stillness.”
St John Cassian
Our incensive power, our internal power of destruction, is of itself good, and is to be placed at God’s service. The issue is that we think our friends (our fellow creatures) are our enemies, and that our enemies (our vices and sins) are our friends. We must study the passions and learn to know and fight our true enemies.
We shouldn’t be kind or gentle on our vices or malicious thoughts. We should expel them with anger. We should dash their heads against the rock, that is, Christ. We must be patient with ourselves, but absolutely merciless with vice.
This brings us back to the question of righteous anger. This anger against our own sins is certainly righteous anger, and I would argue that righteous anger in the larger sense is actually the same thing, only within a community rather than an individual. That is why it belongs especially to those in positions of authority.
It is right for a community to have anger at its own injustices and sins, and so to root them out and purify itself. But it must never be a matter of seeking revenge against others. It must be an act of love for the community as a whole as well as for each of its members, desiring to free us all, and it must be as part of the community. But again, and I must emphasise this, it must be done out of love, and be consistent with love; if it is not, then you are in danger of judgment (Mt 5:22).
How to conquer anger
“The final cure for this sickness is to realize that we must not become angry for any reason whatsoever, whether just or unjust.”
St John Cassian
Anger will always pretend to be justified. Always. No one is ever angry without feeling they have a right to be.
But you do not have a right to be angry, ever. How do I know? Because Jesus Christ Himself, when mocked, slandered, tortured and executed unjustly, did not respond in anger.
If, therefore, you continually recall this with all your heart, the passion of bitterness, anger and wrath will not master you. For when the foundations constructed of the passion of pride are sapped through this recalling of Christ’s humiliation, the whole perverse edifice of anger, wrath and resentment automatically collapses. For can anyone keep perpetually in mind the humiliation that the Divinity of the only-begotten Son accepted for our sake, and all the sufferings that we have mentioned, and yet be so hard and stony-hearted as not to be shattered, humbled and filled with remorse? Will he not willingly become dust and ashes, trampled underfoot by all men?
St Mark the Ascetic
God bless you!