When is anger truly righteous and acceptable?
That's a bit like asking, "When is pollution truly right and acceptable?" If there is a garbage strike you may be justified in putting the rubbish in your back garden but eventually you will have to deal with it. In the same way anger is toxic no matter what the justification. Deliberate, sustained anger is like a wound that becomes septic - we call it hatred. Deliberate, sustained hatred becomes violence and we call it war. Anger is natural and we need to understand it. Hatred, on the other hand, is anger misunderstood. It turns and bites the heart that feeds it.
Righteous anger is a singularly dangerous form of human suffering for the very reason that one feels right in one's anger. When righteous anger takes hold of the heart, when it is encouraged and feed, it eventually takes on a life of it's own. Somewhere down the slippery slope of righteous anger the person, the group or the society need a target for their anger because this kind of anger makes them feel alive. The targets may change but the anger remains. It's similar to taking medicine with toxic side effects. The medicine may initially give a good feeling but in time, as the toxins build in the system, the medicine itself becomes the cause of the sickness.
This doesn't mean that we have to be doormats and endure unacceptable forms of injustice. On the contrary, if we take responsibility for our anger and realize it's harmful effects then our perceptions and judgments will be guided by wise discernment not by deluded passions. Our ability to think and act in ways that minimize present and future suffering will be enhanced because we won't be blinded by destructive emotions.