The emotion of anger has suffered from a bad reputation, probably forever. This is a result of the fact that people who hurt other people are often angry, and no one wants to get hurt. While there has been extensive research on the difficulties that anger can cause, recently more research has been done on the beneficial aspects of releasing anger in ways that are constructive. Unfortunately, anger is all too often treated as a dysfunction.
It certainly can be. If anger is constantly repressed, it can become explosive. When that happens, it certainly is problematic. However, anger is also central to the clinical definition of depression: anger turned against the self, combined with a sense of helplessness or hopelessness. It also plays a significant role in anxiety as seen through the paranoid projection of anger onto the world, making the world look more dangerous than it is in reality. In relationships, conflicts involving anger are the central piece of devolution.
I believe that much of the difficulty that we have in dealing with anger results from not having a good understanding of what anger is and how it needs to be utilized for a healthy psychological functioning. As with all emotions, anger actually make sense if you accept it and then try to understand what it is all about. This is because, while emotions are non-rational, they do make sense. There is always a reason why they appear.
If we define anger in a non-pejorative manner, we can look rationally at how its energy needs to be utilized appropriately to generate a high level of functionality. All problems begin with an experience of injury and pain, the primary negative emotion. Following injury, there are three secondary emotions. Sadness is a grieving of the injury, energy meant to validate and honor the experience. Fear is defensive energy the body generates to avoid further injury. Anger is the natural, healing energy that the body generates to attend to the injury.
Additionally, there is a spectrum of angry behaviors from which people choose. At the "hot" end of the spectrum is what we think of as aggressive behavior, which can be destructive. In the middle is assertive behavior. The "cold" end is passive-aggressive behavior, which can be just as destructive as aggressive behavior. There is a time and place where any of the behaviors on the anger spectrum may be functional and other times where the same behavior can become dysfunctional.
Unfortunately, there is a double bind people learn about anger when it is defined pejoratively as only "hot" end of the spectrum. The double bind is you can either express your anger as "hot" behavior and suffer rejection or not express your anger and not get the underlying injury attended to. The double bind can only be broken by finding functional uses for the energy of anger.
*In terms of treatment, the job of the therapist is to help clients to become more aware of their injuries and their secondary emotions, especially their anger. We also have to help clients to learn how to make good decisions about how to use the energy that their emotions generate. Anger, when defined with acceptance, becomes energy that is meant to be used to attend to your injuries. That means using it for either power or self-nurturance. Power is when anger is used to act on the world so that the world takes care of you. Self-nurturance implies acting directly to take care of your self.
Understanding and utilizing the client's injuries and resulting anger can help inform the treatment process. Once you have identified and accepted the anger that is there and helped the client to do so as well, it becomes a process of working on finding appropriate behaviors for the client in terms of transitioning the angry energy into self-nurturance and/or empowerment.