(Red-letter portions of the New Testament)
Matthew 5:21, 22, " You have heard that it was said to the people long ago, 'Do not murder, and anyone who murders will be subject to judgment, But I tell you that anyone who is angry with his brother (without cause) will be subject to judgment."
Jesus did not say being angry is as bad as murder, He said that being angry (without cause) also makes us subject to judgment.
Unresolved anger can bring under the judgment of God. Unresolved anger leads to harm, to others and ourselves. Cain had unresolved anger towards Abel. It lead to Abel's death and Cain's punishment. Even if righteous anger, anger with a cause, is not resolved, it can lead to harm, evil.
Saint Paul wrote, "do not let sun go down,while you are still angry." James wrote, "man's anger does not bring about the righteous life that God desires."
Even if you have a valid, biblical reason to be angry, lingering anger, even "for the sin, but not the sinner,: can lead to harm. "Sin is crouching at the door. It desires to have you, but you must master it." Do not give the devil a foothold."
Paul writes, In your anger do not sin," and "love is not easily angered." Jesus, himself became angry. Resolving anger is the issue, Jesus' angry response to hypocrisy was immediate, measured, and then over with.
By accepting Jesus, we receive His Spirit, and the "fruit of the Spirit is love, joy, peace, patience, kindness, goodness, faithfulness, gentleness, and self-control." When the love of Jesus overwhelms us and our anticipation for heaven grows, why would we bother hanging on to unresolved anger? That change in our hearts helps us to resolve our anger appropriately and to escape being subject to judgment for it.