“It seems to me that anger is not always a bad thing. Can’t people be energized by their anger, focus, and attain fairness?”
Yes, anger can be part of the motivation for achieving good. Yet, we have to make a distinction between anger within reasonable bounds (the emotion does not disable us, is not extreme) and anger that turns to resentment (a long-lasting and intensive anger that can lead to fatigue, distraction, and even physical complications). If we do not make this distinction, we could slip into resentment and conclude that it is good rather than dangerous in the long-term.
For additional information, see Frequently Asked Questions.