We often interpret this scripture to mean that it is ungodly and unrighteous to feel anger. Anger is actually a healthy human emotion, that helps us to identify danger and threats to ourselves from others. It is unregulated anger, that results in rage and bitterness, which is dangerous and which God considers sin.
So what kind of anger is appropriate and how do we channel this emotion, in a way that is healthy? There is a term called righteous indignation and the anger Jesus felt, when he used a whip to drive out the money changers and people selling animals for sacrifice. “The Scriptures declare, ‘My Temple will be called a house of prayer,’ but you have turned it into a den of thieves!” Matthew 21:13
I am not suggesting that we take the whip, to anyone that we consider, to be showing behaviour that displeases God. What I am saying is, that God expects us to feel anger when there is injustice in the world. This injustice should move us, towards prayer and action.
St. Thomas Aquinas, in the question on anger of his Summa Theologiae, quotes the Opus Imperfectum in Matthaeum, “he that is angry without cause, shall be in danger; but he that is angry with cause, shall not be in danger: for without anger, teaching will be useless, judgments unstable, crimes unchecked.”
God is faithful, forgiving and merciful but he is also a God of justice and vengeance. (Romans 12:19) God also loves justice and hates robbery and wrongdoing. (Isaiah 61:8)
God expects us to be moved, by wickedness and corruption in our world. He does not expect us to walk around, singing and pretending, that everything is ok while, we wait to spend eternity in heaven. We were created for a purpose, and the thing which stirs up a sense of injustice, within you is what you were created to help resolve. It is ok to be angry but do not sin.