A very very interesting question, LadyE :) In my humble opinion (I am a pitiful, namesake Christian) 'Anger' itself is not a "sin" as is usually perceived (Psalms). There are negative connotations attached with 'Anger' because its consequences are usually "negative" and counter-productive.
Psalms implies that believers may be angry, but not 'let the sun go down" on it. Here my understanding is Anger is pure - a sense of justification or a need for justification. But if this pure nature is used as a weapon to seek 'redress' or recompense then it is "sin". The story of Cain and Abel is I believe a very good example.
Then 'Holy Anger' or "Righteous Anger" (The Bible calls it so I think). Here "Righteous Anger" (Or 'Holy Anger') is not necessarily a an emotion. In fact, I understand it is a burning motivation for a right cause, a right objective and it's outcome is only for the good and are good.
If I am not mistaken, there is somewhere written in the Gospel that the Apostles ought to have this motivation when spreading the word of Jesus Christ (or righteous anger). I am not sure exactly where but I think this aspect of "righteous anger" (or burning motivation for a good cause is explained in the Later parts of the New Testament. One good example of this ' righteous anger' I feel is the one when Jesus fashioned a whip and chased out the traders and tax payers in the Temple.
Just my humble understanding of it, LadyE, I am no expert least of all worthy to be making any opinion on topics such as these, but I hope it creates an interest for hubbers to discuss. All the best :)