What is Harry's crime in the Half-Blood Prince?

There is something that has always bothered my about one of the key scenes in The Half-Blood Prince.

As you will no doubt recall, in the later stages of the novel, Harry sees on the Marauders Map that Malfoy is in a boys bathroom with Moaning Myrtle. Harry enters the bathroom and finds Malfoy leaning over a sink, crying, while Myrtle attempts to soothe him.

Harry takes no action, other than to stand looking at Malfoy. Malfoy pulls himself together and looks up into the mirror. He sees Harry standing behind him, draws his wand and casts a spell at Harry. That spell misses and a duel ensues.

The two are well-matched (surprisingly so, considering Harry's experience and training, but I wrote about that in an earlier post). After several shots back and forth, Malfoy attempts to cast the Cruciatus curse on Harry but Harry is a shade quicker, hitting Malfoy with the mysterious Sectum Sempra spell that he found in his potions text.

Malfoy collapses to the floor, bleeding profusely and dropping his wand. The duel is over. Harry takes no further offensive action but merely stands by as Myrtle screams and Snape thunders into the room to render aid to the bleeding Malfoy.

From what I can tell, Harry did nothing wrong. He walked into a public bathroom. He watched a schoolmate cry. He responded to an attack by that schoolmate. He avoided being tortured by that schoolmate (who was attempting to use an Unforgiveable Curse on him). He took effective action to render his attacker incapable of continuing the attack, without actually killing his attacker. He took no further aggressive steps once his attacker was incapacitated.

Where is the crime? Why is he punished? Why is he subjected to such significant vilification from Professor Snape, Professor McGonagall and even Hermione?

Malfoy cast the first spell and started the duel with no more provocation than that Harry happened to be standing there watching him cry. Malfoy escalated the duel by attempting to cast an Unforgiveable Curse. Harry defended himself once attacked and stopped once he had rendered his attacker incapable of continuing to attack.

Malfoy was forgiven for attacking Harry and forgiven for casting an Unforgiveable Curse.

Harry was found guilty of defending himself effectively from an unprovoked, serious, perhaps deadly attack.

Okay, I can understand that Snape, who hates Harry immensely for a variety of perhaps understandable reasons, might see this an opportunity to get at Harry. Fine.

But McGonagall? Once she heard Harry's story and interviewed Myrtle, wouldn't she intervene in support of Harry? Wouldn't she demand that Malfoy, once recovered, be punished (and likely expelled) for launching an unprovoked attack on a schoolmate and for attempting to hit that schoolmate with an Unforgiveable Curse?

And Hermione? Sure, she doesn't like the Half-Blood Prince's book. And sure, she quite properly chastises Harry for using an untried curse on someone like this. But why does she so thoroughly condemn Harry for his actions?

From a narrative perspective, on the other hand, I can understand this scene and its outcome. It provides action in the middle of a fairly static plot. It permits the duel between Malfoy and Potter that we have been hoping for since book one. It creates a reason for Harry to be forced to miss the final Quidditch match and sets up the climax of the Harry-Ginny romantic build up.

And, of course, if Malfoy were punished or expelled at this point, the climax of the entire novel would be impossible -- with Malfoy kicked out of school, there would be no way for Dumbledore to be trapped by Death Eaters on the top of the tower that fateful night.

So I see the value in the duel and Harry's punishment to Rowling as a writer but... the outcome of the duel still does not make any sense in the world of the novels.

Rowling would likely have been better to have the duel start less decisively -- Malfoy and Harry square off, trade words, then begin to duel together. And she should never have had Malfoy attempt an Unforgiveable Curse. It doesn't have much impact in the course of the action scene, it is unsuccessful and it creates the problem of Malfoy getting away with attempting to cast it.

Then, Harry could legitimately face punishment for using a curse without knowing what it did and, perhaps, for responding to Malfoy's less nasty attack with a ferocious spell that endangered the life of a schoolmate.

These are not major changes I am suggesting but I think they would make the outcome make much more sense and not seem so contrived.