God and Mammon aren't the only things people can cling to. According to the Grail Psalter's translation of Psalm 36:5, the sinner "clings to what is evil." (Other translations are more passive; e.g., NAB has "they do not reject evil," Douay-Rheims has "evil he hath not hated.") And you might recall Sirach 27:30 from this past Sunday's first reading:
Wrath and anger are hateful things, yet the sinner hugs them tight.
That's indisputable, isn't it? Wrath and anger are hateful, and the sinner does hug them tight. (And yes, granted, your wrath and anger are perfectly justified, as are mine, but here we're speaking of sinners.)
I think that "hug them tight" gets it exactly right -- psychologically, I mean; I can't speak to the accuracy of a translation. (The Douay Rheims has "the sinful man shall be subject to them," which is also true and important, but makes a somewhat different point.)
This wrath and anger ("ira et furor" in the Vulgate) that sinners hug tight is clearly not rage (or choler), the first of three kinds of anger Aristotle identifies. You don't hug tight quick flashes of anger, unless maybe you're a boxer who can't count on his skill to win. St. Thomas says the sin of this first kind of anger has its origin in persons "who are angry too quickly and for any slight cause."
But in the other two kinds of sinful anger -- sullenness and ill temper -- the origin of the sin is "that anger endures too long," which is where the hugging tight comes in. St. Thomas gives a perceptive description of the differences between these two sins:
Both "sullen" and "ill-tempered" people have a long-lasting anger, but for different reasons. For a "sullen" person has an abiding anger on account of an abiding displeasure, which he holds locked in his breast; and as he does not break forth into the outward signs of anger, others cannot reason him out of it, nor does he of his own accord lay aside his anger, except his displeasure wear away with time and thus his anger cease. On the other hand, the anger of "ill-tempered" persons is long-lasting on account of their intense desire for revenge, so that it does not wear out with time, and can be quelled only by revenge.
From this perspective, the anger I see among Catholics on the Internet is largely a matter of ill temper. And not just because whatever is locked in a person's breast necessarily doesn't wind up in a blog comment. The "intense desire for revenge" among some Catholics is almost palpable.
What to do about this ill temper? From the perspective of the ill-tempered, of course, there is no question of sin, since they aren't holding onto their anger for too long. Fraternal correction, then, requires more than simply pointing out that they are sinning. You have to get them to see that "this long" is too long, and getting ill-tempered people to see something like that isn't easy.
Apart from that, though, I think it's crucial to avoid responding to anger with equal and opposite anger. There are a lot of vices that, when seen in action, tempt you to join in, but anger is one of the vices that tempt you by suggesting indulging in them will virtuously counter another person's sin. (Detraction is another such vice; drunkenness, on the other hand, is a vice whose temptations don't make much of an appeal to virtue.)
At the same time, with virtue lying in the middle, if someone is ill-tempered with respect to something that ought to anger you -- that is, if he was right to get angry in the first place, but wrong to cling to his desire for revenge for so long -- you shouldn't overcompensate by not getting angry, as though his excess and your deficit somehow average into just the proper amount of righteous anger.
All that's obvious enough, I suppose. What I have to remind myself of time and again, since ill temper is one of the vices that triggers a choleric reaction in me, is that it is not the unforgivable sin, nor a mark of utter depravity. Why a person has this vice and not another isn't generally for me to worry about. Everyone is fighting a great battle, as the saying goes, and clucking over how poorly someone else is doing on one front helps no one.