It’s okay to be angry for anger is not in and of itself a sin. Scripture teaches that there is righteous anger. Sometimes Christians feel compelled to suppress all anger in the name of being considered “nice” or “Christian.” Well, we Christians do get angry but when we do we should control it and not let it consume us. As a matter of fact Christian anger at the wrongs of the world have led to Christians triumphing against them.
“Be angry, and do not sin” do not let the sun go down on your wrath. Apostle Paul, Ephesians 4:26.
“There is an anger that is not sinful, that anger at sin, that anger at unrighteousness, that anger at evil, that anger at bigotry.” Chuck Smith, Senior Pastor of Calvary Chapel Costa Mesa, founder of Calvary Chapel worldwide fellowship, graduate of Life Bible College, prolific author, courtesy blueletterbible.org
Should circumstances arise to call for anger on your part, let it be as Christ’s “anger” without sin. Our natural feelings are not wrong when directed to their legitimate object, and when not exceeding due bounds. Thus indignation at dishonor done to God, and wrong to man, is justifiable anger. Let not night and anger against anyone sleep with you, but go and conciliate the other party, though he have been the first to commit the offense.” Let not your ‘anger’ at another’s wickedness verge into hatred, or contempt, or revenge [VATABLUS].” Our Library Commentaries Critical & Explanatory on the Whole Bible, courtesy of biblestudytools.com
He teaches us how to bridle our anger in such a way that, even though our anger is fierce, yet it does not break out, and that it is without delay quenched before we sleep. And this is so that Satan may not take occasion to give us evil  and destroy us. Geneva Bible Study, courtesy of biblestudytools.com
[Anger should] not last at furthest more than a day; that when the heat of the day is over, the heat of anger should be over likewise; and that we should not sleep with it, lest it should be cherished and increased upon our pillows; and besides, the time of the going down of the sun, is the time of evening prayer, which may be greatly interrupted and hindered by anger. John Gill’s Exposition of the Bible, courtesy of biblestudytools.com
Observe, Though anger in itself is not sinful, yet there is the upmost danger of its becoming so if it be not carefully watched and speedily suppressed. And therefore, though anger may come into the bosom of a wise man, it rests only in the bosom of fools. Matthew Henry Commentary on the Whole Bible, courtesy of biblestudytools.com