I think the original authors were trying to get at the idea of mastering the self before trying to master society.
Injustice certainly brings about anger. The question is, whether anger itself is enough to justify retribution. Anger is an emotional response and when acting only out of emotion, many mistakes might be made.
After removing oneself from a knee-jerk emotional response we can evaluate the situation better. If, after the anger has passed, we still feel it's appropriate to fix a perceived injustice(I prefer the term imbalance) then we should act, but anger in itself is not a legitimate justification.