It is legitimate to be angry

We have been taught that anger is not good. It is one of the seven Capital Sins (pride, avarice, gluttony, lust, sloth, envy and anger) which are the sources of many other sins.
Christ is our model. St. Paul advised us to follow his example.
and that Christ may dwell in your hearts through faith; that you, being rooted and grounded in love (Ephesians 3:17).
However, when we read the gospels, we find that there were occasions in which Jesus was angry. They were when Jesus cured the man with a withered hand on Sabbath (Mark 3:1-6) and when Jesus cleansed the Temple, driving out the merchants and money-changers (Mark 11:15-17).
Therefore, being angry is not that bad, after all. Let's listen to Paul again.
Be angry but do not sin; do not let the sun go down on your anger,
and give no opportunity to the devil (Ephesians 4:26-27).
Paul was advising the Ephesians to put off the old nature which was corrupt, deceitful and lustful (Ephesians 4:22) and put on the new nature which was created after the likeness of the true, righteous and holy God (Ephesians 4:24).
Paul probably had the story of Cain and Abel in his mind when he told the Ephesians to be angry but do not sin. "Do not let the sun go down on your anger" has inspired some short stories developed along this line. Again, "give no opportunity to the devil" has also inspired a number of literary work.
From a pragmatic point of view, a disciplined anger is mentally healthy. It is better to practise how to express your anger than to burst into a blind fury when occasion arises.  That probably was what Paul meant in this piece of advice.
Without any preceding cue, Paul suddenly mentioned the thief.
Let the thief no longer steal, but rather let him labor, doing honest work with his hands, so that he may be able to give to those in need (Ephesians 4:28).
Immediately afterwards, he switched back to mention abstract concepts like evil talks, bitterness, wrath and anger etc. Therefore, the very existence of "the thief" was put into doubt. What was going on inside Paul's head when he dictated this passage?
Therefore be imitators of God, as beloved children.
And walk in love, as Christ loved us and gave himself up for us, a fragrant offering and sacrifice to God (Ephesians 5:1-2)
This is Paul's advice when we move to chapter 5. Further down, Paul used no uncertain terms to reject fornication because he thought that sex is always bad. This is no place to criticize Paul, nor his misguided attitude towards sex. The fact that it was mentioned signals one of the problems facing the Ephesian Church.

My dear Advocate, thank you St. Paul for giving me guidance on how to handle my anger. It is OK to be angry as long as I do not give opportunity for the Devil to overcome me. So, help me by granting me Your Spirit so that I speak more edifying words than criticisms. Amen.