One of the most omni-present of the passions is Anger, and I do mean 'omni-present' in that most people do harbor some kind of anger and resentment towards someone else. In the spiritual struggle, it is only second to Pride in being both difficult to completely cure or even handle when it is 'righteous anger.' And, right there, is part of the problem: there are good and bad expressions of Anger, though most of us experience the pathological kind.
What's the difference?
The passion of Anger is one based on fear and resentment. Righteous anger is a force used to repel evil with power that is completely manageable. The righteously angry person may act with violence, but can immediately stop as the situation dictates. He does not 'accidentally' cause collateral damage.
Anger as a passion is a loss of control (as are all the passions). Emanating from Envy, it looks at others and sees what they have but don't 'deserve,' and it becomes enraged in comparison to its own lack. When people 'disobey' us or don't keep the rules we think are important to us, Anger wells up.
In the 'Big Book' of AA (p. 66), it says:
...if we were to live, we had to be free of anger. The grouch and the brainstorm were not for us. They may be the dubious luxury of normal men, but for alcoholics these things are poison.
The 'dubious luxury' of Anger and resentment is one that people frequently indulge in. They allow themselves to judge others, and by the 'judging' I mean condemn. There is a difference between seeing another person's wrongs and acknowledging them, versus becoming angry with that person and hating him. When we condemn the other person, it is an act of the passion of Anger, which does not hope for the other person's repentance and conversion.
You can see now its envious roots: Anger seeks punishment and destruction, not positive change.
Many religious people use God and the Church wrongly to cover over the passion of Anger. They become angry at atheists, hedonists, criminals, heretics, the careless, and others they consider to be 'rule-breakers' who might... GASP!... get away with their sins. Those possessed by Anger talk incessantly about these other people and how horrible they are, and how they will be condemned and punished in hell for all their sins.
Of course, being possessed also by the passion of Pride, they ignore the mercy of God and seek rather to find a way to forcibly convert the other person or punish him. We clergy must always guard ourselves against the temptations of Anger, since it can well up within us and draw us off our actual task: to guide people towards their own repentance and conversion to God.
The passion of Anger has no compassion, no pity, and no love. It is utterly selfish, and this can be quickly gauged by what makes us angry: am I angry because this person is not obeying my wishes or view of the world? Anger that arises from our loss of control over someone or something is unhealthy and selfish. Love respects another person's free will and the right to exercise it, even to his own detriment.
Righteous anger is always an action in the moment. Once acted upon, there is no lasting resentment or bitterness. It is a tool.
The passion of Anger poisons the soul, and boils away even after the excuse has long passed. Therefore, Anger blocks that very important act of forgiveness. Why? Because Anger demands that the other person be lowered to where we are. It wants everything brought down, and even further if we think we are 'innocent'. The problem is that if you think you are at the bottom, it is impossible to go any further down, isn't it? This is why Anger becomes insatiable: there seems no way to punish other people enough.
If we try to forgive, Anger tells us that the person will not only be able to go off in his present 'elevated' status (remember, the person bogged down with Anger senses his own imprisonment, though he won't admit it), but will get to take with his the 'fruits' of his disobedience. He will 'get away' with what he has done.
What does another person's situation have to do with us? My problems are my own, so why should I care if the other person is punished? Will my resentment actually cause the other person to be punished. Anger says 'yes,' bu the truth is that such is not the case. We only punish ourselves by being angry.