"Passions are morally good when they contribute to a good action, evil in the opposite case."
Catechism of the Catholic Church # 1768
All human beings get angry, it is part and parcel of being human. Anger, in itself, is not a sin, it is simply an emotion. Unfortunately, because of our fallen nature it often leads us into sin. We have all heard the expressions: blowing our tops, flying off the handle, or hot under the collar. Anger becomes sinful when we dwell on it and get carried away by it; we fail to bridle our tongue and scream ugly things, yell at our children and act in an unloving manner. Road rage, revenge, and murder....these are all things that begin with anger.
Does anger always have to lead to sin? Of course not. Anger can propel us into positive action, also. It can stir us into taking steps to correct an evil and with God's grace can even be turned into a great zeal for justice. Look at Saint Paul. There are situations where we must, as Christians, use our voices; especially when something or someone is causing physical or spiritual harm to others. This is where anger can turn into a powerful force for good in our lives; when it lifts us from apathy and moves us toward justice. By the grace of God, anger can be a useful tool for positive change. Tempered [lol] with love, it can stir us into speaking up firmly in the face of injustice.
" We ought to speak, shout out against injustices, with confidence and without fear. We proclaim the principles of the Church, the reign of love, without forgetting that it is also a reign of justice."
Blessed Miguel Pro, S.J.