We express our indignation for his sin, but we show no longing for his softening or reconciliation. This is natural but it is not good. As long as there is hope for change, good anger should not only be directed against sin but also be mingled with grief for the sinner . This does not mean that Eskimos at the North Pole may hold a grudge for six months while the sun is up and natives at the equator may only hold one for twelve hours. It means that anger, for all its possible legitimacy, is a dangerous emotion and should not be nurtured into a grudge.
Anger is the moral equivalent of biological adrenaline.
It is good and healthy to experience periodic secretions of adrenaline in reaction to dangerous situations. But a steady flow would damage the heart. So with anger. It has damaged many hearts because it was not put away, but nurtured again and again into a life-destroying grudge. If there is any way that Satan can assist you to hold a grudge he will do it.