I was always taught anger was not an appropriate  emotion to display; it should never be seen.  While I agree it should be a controlled anger, the story of Christ’s anger when he cleared the temple of those buying and selling and the money changers made it sound like He was out of control.  So you can imagine my surprise in finding the complete thought of “do not let the sun go down on your anger”, which was quoted to me many times.   That was the only part of the verse(s) mentioned in the verse “Be angry, and yet do not sin; do not let the sun go down on your anger, and do not give the devil an opportunity.” (Ephesians 4:26-7)  So, we are allowed to display anger and not bottle up, suppressing our distaste and stand up to those taking advantage of us.  Yet in our anger we should not wish or do harm to anyone thus causing us to sin. This thought of “not let the sun go down on our anger” is because if we bottle anger it tends to fester and “bubble and boil” thus increasing possibility of uncontrolled explosive anger and other sinful actions.   And lastly, the admonishment of not giving the devil opportunity because when we let our anger fester, the devil is extremely happy about our state of mind which is in a dark place.