Given that Austen readers tend to be a polite bunch, perhaps only eight-five of them or so would add, "Duh!"
Emma is a novel about moral education, transformation, and growth, though. Suggesting she is at her worst right before she is at her best misunderstands the nature of moral transformation in a way that creates all sorts of critical problems: Why would we think the latest transformation will stick? Is Emma sorry at the harm she has caused or the pain she feels in Knightley's rebuke? Is the damage done to Miss Bates objectively or quantifiably worse than the damage to Harriet?
There are ways, I suppose, one can sidestep these questions. Harriet, we might argue is complicit to a greater degree in her victimization than Miss Bates. Perhaps. Knightley argues that it was the evidence of a corrupting influence that gets him to speak. Certainly. Though that doesn't necessarily mean that he objectively thinks the treatment of Miss Bates is worse than anything Emma's ever done. Often it is the case that we can forgive larger transgressions in friends or loved ones--confident that they are chagrined or feel the pain of their errors--but have a harder time forgiving smaller or more frequent slights.
Reading the novel as a slow devolution of Emma's character leads one--falsely I hope--to the conclusion that the patriarchal intercession of Knightley is instrumental to Emma's happiness and the happy ending. I would like to argue that the effectiveness of Knightley's rebuke lies as much in its timing--Emma's own development has brought her to the point where she has ears to hear--than his power. But I get ahead of myself.
In Chapter Seven of Volume I, Emma talks Harriet out of accepting a proposal from Robert Martin. That Emma does so in such a way that Harriet is unaware that she has done so--or why--is a source of some humor. The finesse with which she handles it, though, blunts the action's ugliness, an ugliness that I sometimes miss because I'm too busy chuckling at her brazenness.
"While you were in the smallest degree wavering, I said nothing about it, because I would not interfere; but it would have been the loss of a friend to me. I could not have visited Mrs. Robert Martin, of Abbey-Mill Farm. Now I am secure of you for ever" (33).
The pain of this revelation is muted for the readers in the realization not merely that this danger (that of a painful separation) has been averted but in our assessment that it was not that close to happening to begin with. For Harriet, though, it was very real, and she must be processing on some level. The subsequent exchanges in which she looks "aghast," says that fate would be "too dreadful" and that it would have "killed" her not to be able to visit Hartfield are easily dismissed as overly melodramatic, but I think we are wrong if we conclude that they are not real.
So, in addition to Emma hurting her friend's prospects, she inflicts, in the wake of doing so, some very real emotional pain.
That's not a completely rhetorical question. The decision has already been made, and Emma's influence over Harriet is such that she has no reason to doubt that she could make the decision stick. Even if she suspected Harriet were on the fence, this trump card might be held in abeyance for that contingency rather than used here to turn a winning bid into a grand slam.
Is it possible that Emma herself has just thought of this? That the prospect of losing Harriet was so distressing she repressed it, thinking only that Harriet "must" not marry Robert Martin without being able to satisfactorily answer, even to herself, why she must not? Such a reading would be consistent with other places in the text, most notably when she admits being puzzled by her own response to Jane Fairfax.
There is a bald egoism in Emma's statements--"It would have grieved me to lose you...it would have been the loss of a friend to me...now I am secure of you..."--that may make us judge her harshly, but there is also a pathetic desperation in it that may make us temper that judgment with some charity. For all the protestations that Harriet is just "a Harriet Smith," a type that is useful to have around, a project to fill her time, the prospect of losing Harriet leads to a very real panic, and that panic ought to key us in to just how isolated, lonely, bored, and trapped Emma really is.
The standard rationalization for Emma's interference here is, of course, that she thinks she can match Harriet with Elton, and Elton is a higher (if not a better match). It's telling, though, that this is not, apparently, the first thought that Emma has. Elton's primary advantage over Martin is not that a match would be better for Harriet but that it would be better for Emma.
I do think that endangering Harriet's prospects with Robert Martin is one of the very worst things Emma does because of that ratio between the (potential) costs of the action to others and the (unrealized but even potentially small) gains that might come from it. If one wanted to defend Emma just a little here, and I do, one could best say that her egoistic blurting of "Now I am secure of you forever" is telling in two ways. It's telling that Emma is motivated by her own selfish interests to be sure, but it is also telling that this motivation is less calculated than sometimes thought and more the product of unconscious (perhaps repressed) emotions.
Part of Emma's moral development is the acquisition of emotional courage. It is hard for any of us (much less those not yet one and twenty) to look at and face the things we are most afraid of, and the things Emma is most afraid of--loneliness, abandonment, boredom--are fears that are not without some foundation. They are fears of things that she has already experienced or is experiencing. Her eventual triumph will be not in manipulating events so that the things she fears will never intrude upon her world but rather in her eventual willingness to face the things she fears and to act in a manner she believes is right.
Even in this chapter, the farther we get from her instinctive panic, the closer we get to something resembling virtue. Yes, the Elton prospect is a rationalization, but it is one born of a charitable spirit that does want good things for her friend.
Yes, Emma's most instinctive actions are often her most selfish and foolish, but which of us cannot say the same? Emma's most calculated or considered actions, especially by the end of the novel, are usually her more noble. Would that we could all say the same to that.