When angry, do not let it become sin.  That is, control your anger.  Pray for the self-control that is a fruit of the Spirit.  Don’t let the sun go down on unresolved anger (Eph 4:26).  Learn to release it by: understanding yourself, understanding the other person, and by kindness and forgiveness.  Don’t let it be planted like a seed that comes back as a weed of bitterness and resentment.  When you have righteous anger over some unjust situation, don’t let it become self-righteousness.