If, therefore, you desire to attain perfection and rightly pursue the spiritual way, you should make yourself a stranger to all sinful anger and wrath. Listen to what St. Paul enjoins: Rid yourselves of all bitterness, wrath, anger, clamor, evil speaking, and all malice (Eph. 4:31). By 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 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 beam in your own eye? (Matt. 7:3).
No matter what provokes it, anger blinds the soul's eyes, preventing it from seeing the Sun of righteousness.... Whether reasonable or unreasonable, anger obstructs our spiritual vision. 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. St. John Cassian