First, of agreement: Yes, absolutely, absence of anger does not guarantee presence of virtue. In my first post on St. Thomas's treatment of anger (both the passion and the vice), I pointed out his teaching about the vice that is the opposite of anger, viz,
Anger may be understood in two ways. On one way, as a simple movement of the will, whereby one inflicts punishment, not through passion, but in virtue of a judgment of the reason: and thus without doubt lack of anger is a sin... On another way anger is taken for a movement of the sensitive appetite, which ... cannot be lacking altogether, unless the movement of the will be altogether lacking or weak. Consequently lack of the passion of anger is also a vice, even as the lack of movement in the will directed to punishment by the judgment of reason.
Second, of qualification: Zealous anger (i.e., the good kind) is not an absolute or virtuous good, something desirable for its own sake. It is rather a useful good, something desired as the means to another good. In itself, anger is a desire to correct injustice and vice under arduous circumstances; what makes it useful is that, as a passion, it keeps us moving toward the correction of injustice and vice in circumstances where we might otherwise give up.
As a means to an end, zealous anger can only be rightly demanded of us -- which is to say, its absence can only be a sin -- when both the end is demanded of us and such anger is a necessary means. I would suggest that, quite often, people demand zealous anger of others (and justify it in themselves) when one or both conditions aren’t met.
The first condition can fail when a person has no particular duty to correct the particular injustice or vice -- as when on-line Catholics get all spun up over poor liturgy in a parish far, far away -- as well as when what provokes anger is not actually injustice or vice -- as when a bishop makes a licit but unpopular decision.
The second condition can fail when the first fails, when a person's duty is not arduous (for example, giving moral support to someone in a diocese far away who is trying to correct his parish's music director), and when a person can meet his duty without growing angry.
The ability to correct injustice and vice without growing angry (in the "movement of the sensitive appetite" sense) has been commended as ideal by spiritual directors throughout the history of the Church.
A third note, of contrariety: I don't find "70% less evil than the other leading brand" type arguments very persuasive, and that's what comments like, "I think they may be doing BETTER than most people who AREN'T getting twisted in a knot," amount to. Sin has enough apologists in the world, and it's no act of charity to excuse sin in another.
Furthermore, the anger I see expressed on the Internet is, I'd say far more often than not, quite simply not anything like what might result directly from "[p]eople who keep abortion in front of their eyes -- what it is in its nature -- and all the disgusting excuses and prevarications engaged in by all sorts of people who ought to know better."
It's a metastasized anger, a reflexive hatred, a habitual posture of derision and spite directed at whatever doesn't meet someone's personal standards.
It's certainly possible that such habits, in any particular person, developed from zealous anger directed against real, radical evil. It's also possible they didn't. In any case, it doesn't much matter.
Fourth, of justification: I've been involved in on-line discussions on Catholicism for nearly fifteen years. In all that time, anger has been a ubiquitous feature. I am not, let me be clear, speaking of anger at abortion or child abuse, or at indifference to these grave evils. I am speaking of anger -- of sullenness and ill-will, and of hatred and derision directed at fellow Catholics -- over things like hymn selection, and whether a priest says "Good morning" at the beginning of Mass.
Faced with this anger, I have at times joined in; at others, reacted with an equal and opposite anger. Often I am still bemused and befuddled by it. Long ago, I learned it was best to ignore it whenever possible.
I stand by what I wrote in that post almost three months ago:
If you don't understand anger, you don't understand a lot of what goes on between people, including between Catholics discussing their Church.
I, of course, am one of those Catholics who discuss their Church, one who finds anger in the words of others and within my own heart.
So no, I don't accept "it's a bit much if we start lecturing them about how terribly they are missing their calling, etc., etc., how dreadful their failings are, and so on," as a relevant criticism of my posts on this subject, nor that they particularly contribute to the tendency of "marginalizing our consciousness of depravity and evil." If anything, I would say they heighten our consciousness of depravity and evil, by pointing out the ubiquitous depravity and evil of hatred and anger in the on-line conversations of Catholics.