If the object of the anger is appropriate (e.g., injustice), then anger is an appropriate response. In the case of reacting to injustice, anger is like an energy that the soul delivers in order to address the injustice with zeal and persistence.
On the other hand, if the object of the anger is inappropriate, then so is the anger. Some examples of this would be anger generated by something petty such as a perceived slight or anger resulting from someone pointing out that we are wrong. In such cases anger is to some degree sinful because its cause is tainted.
Anger can also become sinful if it is inordinate. Even if anger’s object is appropriate, we don’t have the right to rage, strike out, or excessively vent our anger.
Anger is a passion that is hard to master. It has its place, but we must learn to curb it and uses its force for good.
In the video below, there is a humorous illustration of appropriate anger vs. excessive anger. In the first scene, a football player stirs up anger in his teammates (about an opposing team who “dares” to enter their home stadium) in order to win a football game. I know that a football game is a petty object, but allow it for illustration.
In the second scene, one of the players uses excessive anger to rid his house of a mere fly. Although the object of his anger is somewhat appropriate, the anger is excessive and ends up causes a lot of collateral damage. Be careful with your anger, even when it is just!
Scripture says, Be angry but sin not (Ephesians 4:26).