There is no cause for anger
In his book The Sermon on the Mount: The Church’s First Statement of the Gospel, David P. Scaer draws out the true teaching of Jesus, obscured by textual assertions that accommodated man’s tendency to justify himself: there is no place for anger in the life of a disciple of Jesus.
“One who is angry has taken to himself the prerogative that belongs to God alone. The phrase “without cause” does not belong to the original reading. Even if there is a cause for anger, anger must be put aside among the followers of Jesus. There is no cause for anger. Though anger is the prerogative of God alone, in his work of reconciliation in Jesus he has set aside this anger. This makes the offense of anger even more repugnant. By becoming angry the one who claims to belong to Jesus and to know his mind takes an attitude diametrically opposed to God, who is no longer angry. The refusal to be reconciled is the sign that the person no longer belongs to Jesus and from God’s point of view is no longer a member of the community. Here is where excommunication becomes operative.”
Jeffrey Gibbs’ excellent article “The Myth of Righteous Anger” expands on this and is highly recommended.