Anger comes from thwarted expectations, and is a sign of inappropriate boundaries.
Most reasonable and realistic expectations of others are those created through contract/agreement. It is reasonable to expect that when someone exits a previous agreement, they will so inform you, that you may change your expectations.
Of course the highest form of living is without expectations... then there is no cause for anger. However...
Those of us who live in the light have the expectation of not having our free-will violated by others, and thus live in harmony with others of the light but find ourselves angry when forces of evil use force upon us... yielding righteous anger.
The simplest way of handling appropriate anger is to readjust one's boundaries to separate from those who abuse.
Yet there is a higher level... exiting via transcendence the light vs dark game. For every dichotomy there is a transcendent position which accepts the whole dichotomy. Thus even the light-dark game can be seen to be constructed to facilitate growth of consciousness. From this position, what is there to be angry at? Providing you can escape the claws of the beast. Thus... be in the world but not of the world. Being ascended above dichotomy.
The message of Anger is setting boundaries versus what is inappropriate. Appropriateness for the incarnation is honoring the soul which dwells in the heart chakra, though we expect that should also be in alignment with the Higher Self known in the center of the head. With today's emphasis on Neptune, we must look at our inappropriateness and boundaries (what are you protecting from what?) in the context of our beliefs, perceptions, fantasies, illusions, feelings.
So, is your anger about inappropriateness authentic or illusion? are your feelings based on your soul truth or some belief which may itself not be appropriate for you.
Anger may be righteous based upon authentic expectations, the vast majority of which are rooted in contract, while false anger is an attempt to bully others in relationship, and may be based on any illusion we spin around getting our needs met.