Can healthy anger eventually develop into unhealthy anger?
Yes, this is possible. When treated unfairly by others, it is natural to have some anger because this is showing the other and you that you are a person of worth who should not be treated this way. If you continue to think about what happened, and if the anger starts to grow more deeply and pervasively, then you need an outlet for this development. Forgiving can be such a response. If, however, you do not have any outlet at all and continue with the rumination on what happened, then that anger can become so deep that over a period of time (perhaps many months) it develops into the unhealthy kind, leading to possible anxiety, depression, sleep disturbances, and even hatred toward the other. Forgiveness is an important antidote to all of this.